it could just be the port rattling, or something on the back amp panel. I had a rattle once in a 500 I owned, and if I applied preassure with my fingers on the back of the amp plate, the noise went away.
I have been having the same problem with my ep 500. There had been a rattle that was rather anoying coming from the back of the ep500 at certain times.
I checked with my new spl meter to make sure my sub was not running too 'hot', and it did not appear to be the case.
I called axiom and they suggested I tighten the hex screws on the back of the amp plate.
I tightened them all up about 1/2 a turn or so (there were a couple that were pretty easy to turn so I think they may have been the culprit)
Viola- no more rattle.
