I have a strange problem with my car. I seem to be getting a lot of electrical interference on my radio when my ignition is live. If I am sat in traffic with the engine running, and the radio station has some static coming through (as well as music), if I switch the engine off the signal goes as clear as a whistle.

On the drive, if I turn to a preset and the signal is clear, as I turn the key to ignition (to bring on the dash lights) I get sudden interference coming through?

How could these items be connected, the Focus as standard doesn’t have an amplified aerial, and I haven’t chopped any of the connectors on the stereo (this has happened both with my standard ford radio, and my aftermarket pioneer one)

Help appreciated