The ball bearings are definitely very quiet. I once powered up my system without any hard drives, cd-roms or fans(except for the ones in the Enermax PSU) and I could hear it faintly. It's way below the noise level of any fan or hard drive seek noies.
Also there's a thermal sensor that ramps up rpms as needed on the Enermax (at least on the 92mm fan) it starts relatively low(sub 2000 rpms, and I haven't seen it higher than 2400rpms I think) and is even more quiet.
Also there's a thermal sensor that ramps up rpms as needed on the Enermax (at least on the 92mm fan) it starts relatively low(sub 2000 rpms, and I haven't seen it higher than 2400rpms I think) and is even more quiet.