Sounds like an internal amp problem to me. PG is good stuff though, but you never know anymore..Your grounds are fine IMO. Where is your amp mounted? Or is it mounted? I know I have heard of problems with that type of noise, if the amp is screwed down to the metal of the car itself, it grounds the amp frame to the car, that it can cause issues. Just something elso to try i suppose. Most new amps have insulated outer casings though...

13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
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-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)