Psw505 sub Noise at low frequency or high volume.

My subwoofer makes a noise similar to plywood slapping together very fast, a knocking type noise thats very loud during very low bass or when the volume is higher. I first thought blown driver, but I pressed in on it and didn't hear any noise during the travel. It's been like this for some time so my bandaid has been to turn the volume down on the effected sub. The other 505 doesn't have this problem. Purchased on 11/23/12 so I assume no warranty.
Is there anything I should check? I'm fairly novice. Thanks

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