I've installed an audio auxiliary input unit for my car stereo, but I'm encountering a problem. After driving for a few minutes, I start to hear a low pitched "throbbing" sound from my stereo that gets loud enough to drown out what I'm actually trying to listen to.
I got a tip that I may not have a good ground connection - I was tapped into the car's electrical wiring instead of bolting the ground wire directly to a large chunk of metal. Once I remedied that problem I was able to drive for a longer period of time without hearing the noise, but it still came back.
If I turn the unit off and on again, the noise goes away but comes back quickly. If I leave the unit off for a few minutes and then turn it on again, it takes maybe 10 or 20 minutes for the noise to show up again.
What electrical principles are causing this problem, or what can I do to diagnose/fix it?
EDIT: The auxiliary input unit is a separate box plugged into the radio's antenna jack. When the unit is turned on, I tune the radio to a certain frequency and hear music (until the throbbing starts). When I turn the unit off and go back to regular radio, I don't have any problems.