last night at the drive-in movie...

... I noticed noise on my empeg. They transmit the movie sound through a radio broadcast. I received it through PCATS' radio module and heard some background noise that sounded like static. I paid special attention to ground-loops and such when I installed the empeg, but I only tested with the engne running. Now, in the movie theater, I noticed noise with the engine off. There is no noise with the engine on. Before I get my multimeter out to trace the noise, do you have any idea where I should start looking when I have noise with engine off and no noise with the engine on? Thanks for any hints!

I've noticed something similar with my empeg. Ground loop noise when the engine is off, none when the engine is running.

It's still a ground loop, but this time it's due to the change in voltage that happens when the alternator isn't pumping power into the system (I think). Without the engine running, the voltage ends up dropping pretty low. I'll bet that when you turn the engine off and turn the volume up, you also get the occasional blank screen followed by the battery icon, right?

I've heard noise due to the drives spinning/seeking (I think seeking more than anything). I doubt it's getting in internally to the empeg's audio circuitry though. I think it's producing some small current fluctuations in the supply lines that are getting into the amps.

It's so quiet I've haven't ever bothered about it, but it definitely was there. This was in my old car and I'm not sure the M3 with factory amp has it. I think I remember the old car having it even before my empeg when I had an embedded PC with a 3.5" drive in the boot many years ago.

The sound is best described as "fluttering" I think.

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I'll bet that when you turn the engine off and turn the volume up, you also get the occasional blank screen followed by the battery icon, right?

I'll check that. You mean that pumping electrons into the circuit reduces ground loop effects, becasue the current is "pushed" along lower conductances? I'm having a hard time constructing an image in my mind how that should work, I'm obviously not good enough at electrics...