At low voltage, particularly when the low voltage is supplied via PWM, they make a "rumbling" noise. Doesn't matter if they're hard-bolted to a high-mass plate, soft mounted with low-hysteresis material (rubber) or high-hysteresis (50 durometer silicone), held in my hand or sitting on my desk. It's not the same noise at all as that produced by an out of balance fan (which I regard as broken, BTW).
If I had an out-of-balance fan, and if, for the sake of argument, I wasn't going to just replace it with an unbroken one, I'd just balance the thing. If you've got a "dial back to zero" automotive strobe light you can use that to figure out where the heavy point is - or you can just look through another fan that you've got a speed controller on (harder, but do-able). If you've got a CRT, try looking at that with a fan to see what I'm talking about. I used those "inventory sticker dots" as balance weights by putting them off-center on the hub. It would probably be OK to put them on the underside of the fan blades (top surface does the lifting - coanda effect) but I didn't try this.
Bob
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