I'm hoping someone can help me isolate a strange static sound that has been coming from my subwoofer lately. It occurs whether the engine is on or off, just when the head unit is on, and only from the subwoofer. However, no static comes out of the front speakers (both connected to same amp).

Some details about my setup:

Alpine CDA-956 Head Unit
JBL GTO 75.4 - 4 channel amp (104w RMS per channel)
Focal Polyglass 6.5 Components up front (Connected to front channels of JBL AMP)
10in JL W1v2 sub (Connected to bridged rear channels of JBL Amp)

Some of my diagnosis:
-The static is coming from the rear channels, I was able to reproduce the static from a different speaker other than the current JL Sub.
-The static occurs even when all of the RCA cables are disconnected from the amp.
-The static occurs even if I replace the speaker cables connected to the sub and reroute to avoid interference.

The amp is installed in the compartment behind the drivers seat and the ground is securely attached to the metal frame surrounding the opening.

If it were a grounding issue wouldn't it affect both channels of the amp?

Thank you for anyone that can help me shine some light on this.