Alternator Suppression

It's a wild guess, but maybe an alternator suppressor... ?

Another useful way is ground loop isolation, this is a small box that you plug into your RCA cables as it supresses the annoying whine. This along with an alternator suppressor will work wonders.

However, the best way to ensure that you remove this noise for good is a proper wiring job on the audio system. DIY or professional, you can learn about the best techniques for installing audio systems just by browsing the net...

You need to buy a "Ground Loop Isolator", for about ten quid, from any Car Audio supplier.

If you have a factory fitted stereo system, then your problem is due to dodgy earths - check em out.

Do you have an external amplifier? If so try to keep the input feeds to your amplifier as short as possible.