I had this problem with my first ep500 and it was air escaping from around the seal where the amp is attached. Even though all that is routed by machines, if even a little air escapes it can make a buzzing type sound at certain frequencies/levels. When I pushed on the back of the amp plate it would stop or change in pitch. Had nothing to do with wires or internal rattling. Not saying that is your problem, but try snugging up the screws a little that support the perimeter of the amp.
I suppose you could losen them all and put a little sealer/caulking and retighten. Maybe the tooling on the cnc machine needed replaced before they cut out your sub.
