How is it mounted to the case? Are the mounting points isolated with rubber from case-metal?
That makes a big, big difference . . . . just as it makes a big, big difference with cooling fans . . .
If it isn't isolated, there are probably some solutions you can buy, but you can jerry-rig something any number of ways. I'd even seen where someone used rubber surgical tubing to suspend HDDs in 5.25" bays.
Isolating hard disks from case metal is like taking the precision-made sounding board out of a grand piano. Bad for the piano, good for a quiet PC.