Firstly, get your mind out of the gutter.
My desktop has 3 x 750GB hard drives and I can feel a rhythmic throbbing, presumably due to the beat frequency of vibration between the different drives. It's also audible if you listen really closely.
I'm concerned this is going to shorten the life of the drives.
The drives are in a coolermaster "4 in 3" cage which has the drives in a box with rubber grommets between that and the part that connects to the PC case. The drives are not individually "grommetted" which is probably the reason this throbbing is happening in the first place.
Anything I can do? A previous setup with 4 x 300GB drives did not have this issue.