OK, so possibly a bit of a fun one. Haven't had the time to do much troubleshooting yet, but figured I would see if any of you guys have advice on what to look at. The rear passenger speaker has recently (past week or so) started doing a high-pitched squeal. It seems to be directly related to the RPM of the engine, the higher the RPM, the louder and higher the frequency. It is barely perceptible at ~500, but is very noticeable above the music at ~2500. Turning off the head unit or changing the volume on it makes no difference. The sound happens regardless of which gear I am in, and also happens when I am in neutral. It seems to happen if the rear passenger door has been recently (past couple of hours) opened, but goes away if the vehicle has been sitting for a while turned off. Not sure if any of this applies, but I have an aftermarket head unit connected to the factory Rockford Fosgate amp via the pre-outs through a ground loop isolator.