If It have an mechanical pump, It May be due shutdown method: because diesel doesn't have Spark plug, the only method Is to close aspiration with a valve.
When valve Is closed, It Will run briefly in Rich mixture, so It Will create a lot of dust which Will be deposited on muffler.
At the next start, the dust Will be blown out and then, here the smoke.

I've seen lots of LED highbays and floodlights that have very strong 100hz flickering on the camera.

You are quite right, but my main point about waste was from fixtures that contain more than just metal and glass.  Specifically enclosed fixtures for low volt halogen lamps with electronic transformers, and fluorescent fixtures containing electronic ballasts.

A year or two ago a European study showed that the quantity of electronic waste being generated by the replacement of luminaires had skyrocketed from insignificant levels, so now generating comparable volumes of E-waste as the quite heavily regulated industry for white goods electrical appliances.  This has stimulated recent political focus concerning how to reduce the enormous and still rapidly increasing waste stream that is suddenly being generated by the lighting industry during the past few years.

When traditional lamps were phased out, one of the justifications was that there are adequate LED retrofits available for almost all general lighting fixtures and that there would be limited need to replace the fixtures.  The reality has shown that this is absolutely not correct.  The problem is compounded by the quantity of new LED fixtures that are failing long before the published lifetimes.  It is made even more severe by the short product lifecycles.  If a shop installs eg 100 LED downlights and after 2 years 10% have failed, it is often impossible to obtain visually identical replacements for the already obsolete designs.  Or due to natural colour drift of LEDs over time, the new versions look unacceptably different.  This often results in the whole installation being replaced and the 90% good fixtures are also thrown out.  Often this is justified financially by the fact that the newer versions are more energy-efficient, and in countries with very high electricity prices it is actually cheaper to scrap luminaires that are only a couple of years old.

The result is new requirements on the repairability and upgradeability of lighting fixtures - a new industry that is emerging.  Incidentally just last week the UK authorities published a proposal for a new international safety and performance standard governing the so-called “Remanufacture of Luminaires”, which are not adequately covered by the existing IEC / EN standards for LED luminaires.

"It is extremely unhelpful for the customer because now there are certain kinds of fixtures for which it is completely almost impossible to find any LED retrofit lamp via retail channels that will deliver satisfactory service.  Some manufacturers seem to expect the customer to learn that by experience, and throw away the existing closed fixtures in favour of larger or more open or fully integrated LED fixtures.  The resulting ecological problem from electrical waste generation is becoming huge."

According to me the problem is not that much those fixtures needs to be ditched, the fixtures alone do not pose that much of a waste load to the environment, as the recycleability is quite high.

The main waste problem I see here is the need for common customers to find that by wasting many lamps first. These are the main source of an unnecessary waste.

And also I see as quite a waste spending all that effort trying to make a nonsensical arrangement (trying to marry two things that the nature really does not want together, like enclosed fixtures with retrofit lamps) to work. Not that it is not technically possible, but it is nothing else than waste of resources.  Saving a small piece of steel and glass does not help the environment by far enough as not spending all that effort and cost (both environmental, as well as financial, both manufacture/purchase of the required technology, as well as all the R&D going into it) in something that may use way less energy and last way longer.

Angryhorse has hit the nail on the head!  Every year the photoelectric performance of LED lamps and fully integrated LED luminaires gets better - but at the same time cost pressures mean that the lifetime, reliability and general quality seems to get much worse.  Some manufacturers are finding it’s not unusual for as many as 10%-20% of LED products to fail within the first year or two of service, and the warranty claim costs can totally wipe out the company’s profits.  Some of those who continue to offer bad quality products look for caveats to get out of honouring such claims.

It is extremely unhelpful for the customer because now there are certain kinds of fixtures for which it is completely almost impossible to find any LED retrofit lamp via retail channels that will deliver satisfactory service.  Some manufacturers seem to expect the customer to learn that by experience, and throw away the existing closed fixtures in favour of larger or more open or fully integrated LED fixtures.  The resulting ecological problem from electrical waste generation is becoming huge.

Even in the fixtures world this is becoming a big problem.  At a time when we have to insulate buildings better to reduce energy costs, a very common logo found on the technical datasheets of ceiling recessed fixtures, panels, downlights etc is that it’s not allowed to cover them with insulation any more!  Otherwise they overheat and burn out quickly.  Few customers realise at the time of purchase that they have to cut big holes in their roof insulation, even up to eg 20cm all around the perimeter of the fixture.

Another increasingly common trick on ceiling fixtures, track spots etc is to state “max ambient temperature 25C”.  To most people that sounds OK since typical rooms are cooler.  But heat rises, and at the ceiling height or in ceiling voids the usual temperature is closer to 30C and in summer even higher.  I know of many fixtures from formerly big brands where even safety can be compromised - for instance plastic or electronic components or wiring insulation might be running actually at the maximum safe operating temperatures declared on their datasheets, at ambient 25C.  However at ceiling temperatures they know this will be exceeded, but claim immunity by stating that the customer is not allowed to use at higher and even typical or expected ambient temperatures.  The laws on “expected use conditions” are not especially robust and open to interpretation, and many choose to ignore what is to me an obvious safety or reliability risk.

In Europe it’s now compulsory to state the actual maximum allowed operating temperature (unless that is only 25C, in which, ironically, no marking is required).  A few of the better companies allow significant headroom - for mine we invariably state that the fixture is safe for use at up to eg 45 or 50C, and driver life calculations are made not only at 25C, but also at the maximum rated ambient temperature. 

This ensures far more reliable products - but also slightly more expensive, and it is not always an easy sell to persuade customers of the value of selecting fixtures with adequate thermal headroom.  Slowly though the market seems to be learning.  But I consider it unhelpful when certain far bigger companies are so desperate for immediate sales, in an effort to halt their financial failings, that they continue to push the cheapest products that are bound to fail quickly (usually alongside a professional range for the few customers that know how to identify quality products).  The only hope is that over time the irresponsible companies will damage their reputations by such frequent failures that customers begin to look for the better brands.  In professional channels I expect the situation to regulate itself, but for retail sales and home lighting, the LED lamp and fixture market has become a minefield of horrendously unreliable products with no way for the end consumer to identify the good products from bad (other than slow experience by brand name).

I haven't noticed flicker being an issue myself with most LED, but I have noticed many places have replaced their fluorescent troffers with LED edge lit troffers and a month later many of them are already eol, yellowing and unevenly lit

As for air release sound, large diesels are often started using compressed air. That saves the need for an enormous starter motor and battery. Air is just stored at rather high pressure and entered in the cylinders in a sequence using special valves. Also there may be some starting aids operating pneumatically.

It depends on multiple factors:
Too much fuel for not enough air:
How the governor works, mainly how much fuel it injects during start (simple mechanical ones just set full injection dose so it starts and spools up rich until it reaches set rpm, more complex hydraulic ones tend to slowlu increase fuel dose, so unless the cranking takes too long it starts with small dose and spools up before the dose increases, most modern electronic injectrion control is able to inject just the correct amount to not yet create any excessive soot).
Also how the max dose limitter is adjusted plays a role, or even whether it is made to follow the charge pressure (mainly with turbo charged engines, which get way more air pressure when running vs when starting with turbo still stopped, while the fuel dose limit uses to be set to the running pressure, so with turbo spooled up),

Improper burn conditions
Does the engine feature glowplugs and are they working/preheated during the start? Without them the fuel burns too long and is not atomized that well when cold, with the higher injection dose at spool up it leads to incomplete burn.
Slow moving injection pump cam may lead to too low injection pressure, so also improper fuel atomization.

Except the most modern engines, the smoke on start used to be tolerated as it was supposd to happen only for a brief moment.
But once the emission standard tightened, mainly requiring the use of things like DPF, the engine management had to be done so to prevent the smoke even at start (as that would clog the filter way too fast)

As for why one engine smokes at start and second don't, these may be from different generations. Older engine may be worn with worse compression and more oil leaking into cylinders. Also older engine might have a mechanical fuel pump which delivers just a fixed fuel dose per rotation, having sub-optimal conditions a start, and newer engine may have electronic common rail. Common rail systems get quite sophisticated, running at about 2000bar (!) fuel pressure for precise fuel atomization and having a powerful microcontroller opening injectors in a programmable way. So, on start, it may wait for crankshaft RPM to stabilize with no fuel, and then begin gradually opening the injectors until engine pick-ups, managing a smokeless smooth start.

Some physical/chemical facts for the beginning.

(1) Motor fuels are hydrocarbons, mainly composed, as said, from carbon and hydrogen.
(2) Diesel fuel has notably higher carbon to hydrogen ratio than petrol.
(3) Carbon rich fuels need more mass ratio of air to burn completely, to clear carbon dioxide and water, as carbon takes two oxygen atoms for on carbon atom, and hydrogen takes one oxygen atom per two hydrogen ones.
(4) Diesel fuel has notably higher boiling temperature range than petrol.

That said. As carbon rich fuels have more carbon and need more oxygen to burn, they tend to start to soot easily when burned incompletely without all the oxygen needed. Then, when engine is first started, it sure runs at sub-optimal cycles. For low RPM, compression will be poor and air dose will be lower than usual. Also, engine does not start immediately and fuel doses injected on previous cycles will partially stay in the cylinders making excess fuel. As diesel has high boiling point, extra fuel will stay for long on the cylinder walls. All that makes for low oxygen and too many fuel, making the engine to soot for some time. Also, when engine is stopped, some oil enters the cylinders, acting then as a fuel and taking some oxygen to itself. Even heavily worn petrol engines can soot because of this.