My hostel have a new standing cordless vacuum cleaner, which don't have the usual blower noise of most vacuum cleaners, but more like that of a piston pump that used in pressure washers.
Why?

Don’t know! That’s a bit weird! And even slightly scary 😟!

Possible I suppose?, they can make piston pumps to pressurise water, why not to create a vacuum?

Trust me, that is not what is happening. Why wouldn't you use pistons? Because those are low volume, high vacuum pumps that also cause a ton of vibration. Also way way too expensive to put into a 50 to 500 dollar device.

It is more likely that it has worn out bearings making it noisy, or have a bit of debris stuck to the fan throwing it off balance. Other things that can make it sound differently are BLDC motors instead of series wound universal motors, the former have a more constant motor speed, and the absence of commutator whine in a BLDC motor.

Actually, some displacement pump like rotary vane may be in use! Why? Turbines struggle to get really good vacuum and do this with poor efficiency, especially small ones. Look at a regular vacuum cleaner motor-turbine. Rather complicated mechanics here, including multiple turbine wheels in series. And it eats up to 2kW to get the job done satisfactory, modern 'green' models suck literally))

Or something stuck around the impeller 🤔

The noise hears more like that of a piston pump.

Update: I wanted to make a video of it working, and soon discovered that the said noise is only my hearing illusion: The piston pump like noise comes from the brushes and it have the usual turbine sound.

Actually, some displacement pump like rotary vane may be in use! Why? Turbines struggle to get really good vacuum and do this with poor efficiency, especially small ones. Look at a regular vacuum cleaner motor-turbine. Rather complicated mechanics here, including multiple turbine wheels in series. And it eats up to 2kW to get the job done satisfactory, modern 'green' models suck literally))

The modern 900w models of good brands designed in EU are actually really quite good. My parents had a 2kW AEG and a 900w AEG, they perform equally well, except the 900w one is much quieter. Sometimes you can find actual suction power specified in watts. A 2kW aluminium wound (for cheapness! Sacrificing efficiency) vacuum cleaner motor can perform worse than a 900w copper wound motor.
Brands like Nilfisk, extensively used in professional cleaning, kept using their 900-1000w motor units for years into the era during which consumer vacuum cleaners reached 1500-2000w.

For energy consumption it's not a big deal imho, vacuum cleaning takes maybe 15-30 minutes per week? So in that regard i don't care about the regulations. But what was a very nice side effect, is that vacuum cleaners have gotten a LOT quieter. Though there are rules about that too, limiting noise to 80dB or so.

What works like crap are things like a bagless 600w Philips (Oh philips, how far you've fallen) designed and made in China. That thing performed on par with a 500w Holland Electro from 1960... I've had 2 bagless ones, both were crap.

I don't think they use a rotary vane, just a smaller but higher RPM centrifugal pump/turbine. Multistage vacuum cleaner motors aren't something special, aforementioned Holland Electro already had 2 and 3 stage fans in the 1960s. I haven't disassembled a modern, well performing vacuum cleaner motor yet.

If it truly sounds like a piston pump, there's only one explanation: it is broken, or in the process of breaking. I've had a Hoover Junior from the 1940s of which the rear bearing bushing was worn, and it made quite a nasty roaring racket. Usually those old hoover motors run pretty smoothly. 

Actually, rotating brush may be a source of low frequency noise in Dor's case.

 

Or something (some piece of paper, plastic film,...) stuck in the tube could be fluttering there and create the noise. And once tried again, it got freed and so the noise stopped...