do the newer 500's even have a ground screw anymore? Unless the sub is picking up some type of interference from something nearby, although unlikely, it still sounds like the amp itself, if the power chord is the only thing plugged in to the sub when it makes the noise.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85