My new Polk Audio 505 subwoofer rattles, almost as if something is loose inside. It's as if the entire enclosure is vibrating. It doesn't occur all the time; only on certain base passages. The volume is set at less that half. The vibrating doesn't go away until the volume is so low that I'm getting no bass from it. It's sitting on a hard linoleum floor. Would placing it in a soft pad help? Would turning the crossover down from 100 hz to 80 hz on my Onkyo 606 help? (I have relatively small fronts and centers; that's why I had set it at 100 hz). Could the unit be defective? The other bit of info.: I can localize the low frequencies as coming from the Polk rather than being diffuse as I've experienced from other subs.

Make sure your sub cable is properly plugged in to the jack (snug). It could also be a ground loop.