hi,
i just bought a 96 M3 about 3 weeks ago. i noticed that there was a good amount of alt noise coming from the left front channel (can't tell which driver is making the noise) i took out the head unit and it still happened.

i have since installed a new system consisting of the following:

alpine cda-9815 HU
JL 300/2 front amp
RF 1000.1bd sub amp
Focal 130kp i think. they're the new Polykevs
2 JL 12W3 v2's

i hooked up all RCA's going through the middle console, and bear in mind that most of this equipment, except the power wires and the speakers, came from my old car. never had noise.

i disconnected the factory midrange and took out the woofer and tweeter for the focal units. and i used the factory speaker wires because the previous owner had someone hack into them and take out the rear factory amp. so i took the MUCH easier way and used the factory wiring.

i do not know what to do now. the HU is using a harness connected to the factory HU harness, all power is running from the amps, none from the HU, and i'm only running frontstage, no rear speakers.

so now, when the old and new radios are on, off, and not even there, i still get alt noise in my front left channel. and it's very annoying.

P.S i noticed a ground wire that is in the group of wires that used to connect to the factory amp. it's there with a 12V live, they aren't connected to anything. would grounding the loose ground wire help at all? does the harness ground the radio in the back with the factory amp or what?

i'm sorry my thoughts aren't sorted in an understandable way, but it's been a long day and i'm tired.

thanks.

Make sure your running 8gua 12V and GND.

and now for the final trick..

iv had the same problem.

Do not screw your amp onto the chassis.

if that dont solve it.

try running some new wires.

i used to get the same prob.. a mild alt whine on my left driver TWEETER.

im sorry to say.. if that does not work.

Take some time and re-wire your system.

It is not Your RCA. Try unplugging it and see if you still got it.
when i took my old amp out (Alpine/Nokia)
i swapped it with a Pioneer amp.

Got mild alt whine from left tweet. (Tweets where running off of amp)
you might have your whole left channel whinning.

all i did was Un-screw my screws holding the amp into the chassis. and made my own spacer with 8gua wire tube. so that the screw dont touch the amp.

thanks for the replies guys, but i forgot to mention that the amps are not bolted to anything just yet, they're just sitting there velcro'd on the carpet until i have enough time to build an enclosure and a rack.

any other suggestions.