Since one possible explanation of the  noise is dust, I used the particle counter to quantify the dust.

Instrument name: MET ONE Airborne Particle Counter HHPC6+

Acquisition time: 10 minutes

Volume: 28.36L

Inside the clean booth of Tama central room.

Almost the same order of a normal room. Indeed I realized that the fans of the clean booth were OFF since a couple of month ago, when I checked whether the acoustic noise was important.

I switched ON the fan and wait half an hour

After one hour

The morning after I came inside the clean booth and run the count

So it looks that an entire night doesn't clean the air much more than an hour.

Moving very gently I repeated the count

I think there is some dust on the everything, so when I move the air, the dust flies and make the count higher.

Then I moved the particle counter in the small clean booth on the small optical table that is used for gluing work

Very clean...

And then I went to clean room of ATC