Tip: How to make the NUC really quiet

I have a DN2820FYKH, and with the BIOS setting for the fan of 20% it's really quiet.

But in a quiet room, a rather high-pitched little noise is still there.

Now I discovered that this is because the top plate of the NUC chassis is acting as a resonator and amplifying the fan sound!

Simply put something (not too light, not too small and not too big, it must exert a bit of pressure on the top plate to stop any vibration) onto the NUC. Bam, now it's REALLY inaudible except when you put your ears a few centimeters from the device!