Thump in subwoofer

I have the Outlaw LFM-1 Plus subwoofer connected to the 976 RCA Sub Out connector. With the 976 and the power amp in off mode I get a very noticeable thump out of the subwoofer every few hours or so. Any thoughts on what may be causing this?

Do you have the power switch set to auto?
That can sometimes cause that because the amp doesn't a strong enough signal and it shuts itself off.

Yes, I've had that problem albeit with different components. The problem was caused by a ground loop combined with dirty power. There was enough of a ground offset that the sub would occasionally turn itself on producing a noticeable thump.

Are your components on different circuits? Do you use a power conditioner?

I actually had the subwoofers on "ON", have switched them to "AUTO" and I don't believe I have heard the thump in a while. This doesn't make a whole lot of sense but I'll keep an eye on it.

Thanks for the "ground loop" idea. Had not thought of that devil. The Amp is on its own circuit so there is that potential problem.