That looks like a really shoddy ground, Clean that paint up, bare metal, use a nice big washer and tighten that bitch down. Make sure the power from batt to amp is big enough as well, 4gauge is probably fine for that setup

 

8 gauge is more than enough for that amp. 4 gauge would just become a pain in the *** to get in the terminals.

It sounds like the sub has a bit too much power or is playing well below the tuning frequency of the enclosure. Is it only on the really deep bass notes that it does this?