isolating a hum coming from my tower? (that I cannot hear)

This is a serious personal hobby computer, not work computer.
music is my serious and expensive hobby.
when large capacity SSDs become affordable I'll get one, but until then the mechanical drive has its use.

 

Slow spinning drives are the easiest answer, laptop jobbies normally even better. It goes against the grain not to get the fastest drive you can, but for music work, I doubt it'll even result in a performance hit. Seagate 7200 drives are the loudest, most vibration friendly HDDs on the market.