I once tried suspending the whole drive cage, by putting it on a layer of foam at the bottom of the case. The problem here is that hds vibrate, and the drive cage, when it isn't attached to something, will vibrate and produce a very low pitched hum (at least my drive cage did).
The problem with such a hum is that acoustic foam won't absorb it (too low pitched).
But that was *my* drive cage, maybe yours won't do that. You should try it anyway, it won't take long.
I suspended my entire drive cage too. I just zip-tied it to the 5.25" drive bays above it, and left a little bit of slack in the loops. It's not resting on top of anything at all (no foam). It dampened the noise from the seeks a LOT. It was incredible how much the noise decreased. I haven't noticed any "humming" from my setup.
However, I only have one drive in the cage, and it's a 5400rpm drive. Faster/more drives might (probably) make much more noise.