This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
There does not seem to be a problem with either the head unit or the
amplifier, and the car's charging system is suspect. To see if this is
the case, we can use a system in a car that is already known to be
"quiet." Bring both cars together as if you were going to jump one,
and use jumper cables to connect the two batteries. Start the engine
of the car with the noise problem, and listen to the "quiet" car's
system. If the noise does not go away, there is a SERIOUS problem with
your car's electrical system (possibly a bad alternator). Have a
qualified mechanic check the charging system out. If there is no noise
in the "quiet" car, then the "noisy" car's charging system is
definitely quiet, so continue with level 5.