A very common topic on the Mp3Car Forums is getting rid of strange audio noise (typically synchronous with the rev of your engine); an odd humming or whining sound. This is generally caused from using multiple ground points in your setup. Because these points are connected to a common ground but from different locations, unsteady or irregular voltage may occur. We can separate the grounds from each other by using a Ground Loop Isolator.
Here is an old one from the archives-
ground loopForum member Gip shows us how the poor man constructs his own Ground Loop Isolator. He uses two transformers removed from a couple of old 56k modems, an RCA adapter and some breadboard. Put it all together and viola... no more audio noise. Originally posted as a means to "remove power supply noise from audio". See more pictures and a full explanation on the thread.
In addition to using a GLI, it is also preferred to use high quality shielded cables, and to keep any power lines away from your signal lines - Make Oersted proud!
For further information on this topic consult the Car Audio section of our forums or see the Induced noise FAQ
If you have alternate solution to problematic audio noise you feel should be recognized make a post in the Mp3Car Blog Talk section.