2000 durango problem w/ subwoofers

Hey everybody. Long time no post. I just bought this truck and I decided I would rip the stereo out of my old car and put it in the new vehicle. The system consists of a Pioneer head unit, two Pioneer 10" subs in a sealed box, and a Sony amplifier. This system worked flawlessly in my other car, and it was only in storage for about 2 weeks before being installed into the new vehicle, and for the most part - it works almost flawlessly in the truck.

My problem is this... For some unknown reason, regardless of input source, volume, or anything else... Sometimes my subwoofers just start to oscillate for no reason. It's quick too, almost like machine-gun fire, and it will start low, get louder, and then disappear completely.

I installed the head unit first, with little to no issue, using a Metra wiring harness and dash kit.

This truck does have a factory Infinity sound system... So I spliced both the factory amplifier and sony amplifier into the same remote turn on wire from the head unit. Could this be a problem?

Troubleshooting so far? I have checked all my wiring, and it seems to be good, including making a new ground for the amplifier. (I usually just use one of the bolts holding the seat rails into the car) Today I pulled up the carpet and sanded down a section to bare metal and drilled a hole and used a new screw and washer to bolt the ground cable to. My ground cable is only like 18 inches long. This did not fix the issue.

When the subwoofers were rumbling for no apparent reason, I pulled the RCA cables right out of the amplifier, and the noise continued even with no signal. Only the power and ground wires and speaker wires going to the subs were hooked up.

The music plays fine and my subs respond normally most of the time. It's just annoying when I'm listening to the radio at a reasonable volume and then my subs start to rattle around for no damn reason.

Is my amp dying? Or? Can anyone shed some light on what else I can try to rectify this issue?

So... After looking over my setup again the other night... I noticed the low-pass filter and subsonic filter switches were both set to -24dB, and I actually have no idea why that would cause problems? However... After setting them both to -12dB instead, the annoying interference has gone away.

If someone would care to explain? If not that's cool... I'm just happy my doodie works.