Maj, the sounds that you've described wouldn't appear to have any relation to a ground loop, which is a continuous relatively low level hum not related to changes in the program content.

The receiver sub output or the cables also wouldn't account for what you describe. The poster at MonoPrice was simply saying that after he ran the coaxial cable behind the wall he connected the receiver and sub with short lengths of the same cable.

From the information that you've given, an internal defect in the sub appears to be most likely.

-----------------------------------

Enjoy the music, not the equipment.