Constant "thumping" from amp/sub

[aesic]

I wasn't exactly sure how to search for this so if someone could give me some suggestions or point me in the right direction I'd appreciate it.

My sub/amp constantly "thumps" at regular intervals. It is very annoying at normal volumes and is only masked when I turn up the volume and on the highway. The thumping does not change loudness or intervals when I rev the engine. However, I just figured out that the intervals between the thumping is a lot longer when the volume is very low, nearly inaudible.

Is this a grounding problem and if so, what isn't ground right? I removed the back seat (240sx) and have the amp in its place, ground to the bottom seatbelt bolt. The HU is currently not mounted in the brackets because I will be installing a DIN cupholder/change holder. Will mounting the HU on the brackets get rid of this thumping? Its just been sitting in the compartment for a while because I haven't had time to install the DIN cupholder and because I wanted to fix this problem before I get everything mounted. If it isn't the HU, are there any other suggestions? Thanks.

[PoorManQ45]

First thing I thought when I read this was, well duh, it's a sub, it's meant to thumb

:roflmao

Seriously though, it kind of sound like your HU is not grounded and the preamp(RCAs) are grounding through the sub amplifier

[aesic]

What is the proper way to ground the HU? I noticed a grounding wire that is bolted to the metal by the shifter but I don't remember it ever being connected to previous owner's aftermarket HU. I have this wire touching the metal outside HU but not connected to any HU wires or anything. Will mounting the HU in the brackets and putting it in place be sufficient for grounding it or do I need to do something different with this apparent grounding wire?

Also, I don't quite understand what you mean by the "RCA's are grounding through the sub amp". Do you mean that they are grounding through the HU or through the amp?

Thanks for the response

[Silvia2b]

The first thing I would check is your speaker wire at the amp, make sure that the connections are solid, not loose, and no wire strands touching the other terminals. This could also a loose wire in the subbox on the speaker terminal

The other thing I would check is that the RCAs are not touching any metal in the dash.

[stoy74]

yeah rcas get some black electrical tape and wrap them it will also keep them from comein loose and pulling apart. make sure all wires are not nicked in anyway if so you can use some fingernail polish on the nicks for a short term fix.

[aesic]

Thanks for the help guys. It was just a matter of mounting the HU in the brackets and screwing it into place. Before it was just sitting in the console and not mounted. I wish I had taken the hour it took to remount it instead of suffering for 3 months with a constant thumping or the music turned off. Good lesson to try the simple solutions right away.

[PoorManQ45]

Woot, I rock. I got it right on the first try

Like I suspected, that thumping was the HU grounding itself through the ground on the RCA cables