Hi there, I've got a 99' 740il completely stock. Since last year there's a weird audio hum or static that's quite loud from the speakers when the key is in the ignition but the engine isn't running or hasn't been started. Turning on the radio produces normal radio audio on top of the static. I've been unable to create a scenario where the static would go away.

Does anyone have any recommendations of items to check for troubleshooting? radio headunit, amplifier, etc?