I havent really gotten a chance to try and fix this problem yet. So I thought some of you guys in here can point me in the right direction.

I have an old Rockford 4 channel amp that is powering my front stage and my subwoofer. I've been taking the sub in and out because I go mt biking sometimes and need the space. Recently I just put the sub back in and I noticed that the bass is acting funny. When I turn down the volume on the deck all the way to ZERO, I can still hear my subwoofer rumble. It is not playing a specific song, but it just kinda rumbles lightly.

From what I quickly saw, the ground on the amp is fine. And something to add, my deck is pretty old and is looking at its last days. Some of the buttons stop working at times. So I am thinking this could be a problem with the HU.

I do not have access to a spare amp, nor a spare HU.

Thanks.