I've had the grounds connected to the lug were the negative cable from battery connects with a crimped and shrink tube lug with a 14 gauge wire I ran to HU and spliced with factory ground.I have moved grounds from battery to factory amp ground on left side and even sanded to bare metal and connected to bolt on factory amp rack.
I tried with second ground disconnected
The amps are mounted to sub box and I ran a jumper wire from amp chassis to ground
I swaped amps thinking internal grounds were bad(I have never had a problem with a p series amp)
I laced all the RCAs on the back of HU thinking internal ground in it was bad
The strange thing is I don't think it is alternater noise because when you give the car a little gas it goes away its only when car is idling
What runs in that left side?
I took factory amp and sub out and tied into speaker wires for speakers from there
The only way I could run them further away is take out seat and run them through middle of floor pan,but I would have a hard time believing that's the issue not unless this is just a bmw thing
I guess I am going to buy 3 new sets of RCAs and go from there
Thanks for all the input