Whenever I start the F96T12 IS Slimlines in my basement, one of the fixtures almost makes a "whirring" sound on startup, which is accompanied by a near-strobe effect in the lag-side lamp. This subsides after around 1-2 minutes of use, but the ballast still has an audible hum, albeit a more mellowed tone. Are there any possible explanations for this? The lamps I am running are F96T12/CW Supreme, made in Poland.

I have another fixture that does not flicker at all on startup but it contains my Cool Green lamps from Westinghouse. That ballast is generally quieter also, if there's any correlation...

The lamp is probably rectifying. Try to to replace it with other knowingly good one.

@RRK - OK, thank you. I will try that.

Is the lamp have blackening at one end? How the flickering looks like?

@dor123 - No blackening, no flickering after warmup.

My uncle has a 2017 US-made Philips C50 Supreme that does this in one his slimline strips in his garage. It has always done that when powered up after staying off for a while since day one of installation back in 2017-2018. Still works just fine, stops flickering after a minute or so, and has no blackening.

Not sure what it is.

@fluorescent lover 40 - yep, it's exactly that phenomenon.

My uncle has a 2017 US-made Philips C50 Supreme that does this in one his slimline strips in his garage. It has always done that when powered up after staying off for a while since day one of installation back in 2017-2018. Still works just fine, stops flickering after a minute or so, and has no blackening.

Not sure what it is.

It is hard to tell without actually seeing what is going on, but I expect some asymmetry. Slimlines are instant start, so may be cathode spot initially forms on a poorly-emitting part of the filament causing some rectification, but lately migrates to a better part, so the tube works normally. When this happens, you may see some flickering and movements at one of the tube ends.

In the extreme case, one end can completely fail to go thermoionic so the tube mostly conducts one way until this ends filament finally heats up by electron bombardment and current passing and finally starts to emit electrons.

Interestingly, this can not happen on the leading branch, as capacitor blocks DC completely, and if the tube is rectifying, significant DC offset voltage  will form within a few line periods and the tube will either extinguish, or the lazy end will be forced to ignite properly, too!