Very strange indeed.
I started with two 300GB Maxtors for data and one WD250 as systemdrive in an Antec P150. The two Maxtors hummed like hell. When I put my finger on them I could also feel the vibration and it wasn't good vibrations. How's that with your WD? The WD should not have that problem, mine doesn't. I guess the Maxtors suffered from a misalignment of sorts, send them back to the store and got OK new ones. Perhaps you can do that too. I could have invested in HDD silencing, but I think it is best to work at the source of all evil.