This is a question of curiosity. I have a '98 Acura Integra, a cheap car stereo with a 3.5mm aux in, a cheap inverter and a smart phone. If I have the smart phone audio hooked up to the car stereo and the inverter charging the phone I get a high pitched tone through the speakers at around 4k RPM that increases in pitch with the RPM of the engine.
I know the solution is to not use the inverter, there is no reason to be doing that, but I'm curious why the pitch of the noise fluctuates with the engine RPM. Shouldn't the output of that DC outlet be mostly consistent?