Last year I bought a very noisy (unintentionally) external 16TB HDD for my TimeMachine backups for my main Mac.

That was when I learnt about sorbothane hemispheres, which did a lot to dampen the drive noise for me, but not perfect. Over time the leftover noise in my office has been driving me nuts and this morning I was about to pull the trigger on a $800 replacement drive when I suddenly wondered whether it was more a problem of the drive sitting on fairly thin particle board. Put a thick notepad underneath it and ... silence. $800 saved. Sometimes the cheap solutions are the best.