Weird thumping with sub

[krazeyeyez]

Recently my sub has had like a weird knocking sound, the level of bass when it happens sounds almost like someone is walking on the floor above me but they are not, i am pretty sure it even happens when their is no signal or the sound is muted. It is sporadic and if i unplug it, it seems to stop at least for a while.

Any ideas, wasn't sure if this was some kind of feedback or something. Its very faint when i am watching or listening to something so i have let it go for a while, but recently it has been driving me nuts.

Paradigm ultra cube 12
Integra - dtr 6.8
hooked up with the sub out