Thumping Sound in Sub

I do not know a lot about stereos but would like to solve this annoying "thump".
I have a Kicker 12 run by an SX6503. They are within 12-18" of each other. The tower and interior speakers are run by a KX8504. There is a sub volume control and the "thumping" gets louder as I turn it up. The only other items in the boat when on is the depth sounder and XM radio. Head unit is MB Quart.
Annoying as .... Any suggestions??

I think subs are supposed to "thump"...... sorry had to

sounds like a ground issue with the amp...what are your crossover settings..

Subs are supposed to thump- mine do and when I turn them up, they thump louder. I guess I could not resist too.

My money is on the depth finder. Cables are probably to close to the transponder cables.

a mouse crawled in your sub and is bouncing around. LOL

HAHAH swami lol

Ground touching or loose/damaged wire.

"depth sounder" as you put it. Does the thump happen at regular intervals? If so, that is more than likely your issue.