Very noisy! And worst part is that they are usually "many", so the compound effect sometimes adds up quite a bit, depending on the frequency.

But there are even noisier fans (luckily they are bigger so, you would never have too many of them distributed on small places like the ones above)... the 3-phase fast (4500-5500 RPM), 30 cm ish diameter industrial grade for cooling the water cooling racks, can be even worse (85dB+).