Mainly just cheap PCs, but not all. We have an expensive HP server that sounds like a Hover. If you look at the Dell's web site you will see they have different ranges of PCs Low, Mid, High. You can select the same spec in the Low and Mid range, but have a €150-200 difference. The difference is the Chip Set (bus speed), Memory speed, Hard Disk speed/interface (when SATA was new) and of course the quality of the case/PSU and thus how loud it is. Then you have the Gamers with high speed graphic cards which can cost as much as the base PC.
Where is an industry around reducing PC noise.
Towger