Hi
I'm not sure this is quite the same problem/solution but...
I noticed that my radio reception was getting gradually more hissing, but was better with the ignition off than on. It turned out to be a deteriorating Earth connection - the small bolt through the smaller wire, going from the battery negative to the car body just in front of the left-hand headlight.
Reception was restored by wire-brushing away the rust from the bolt THREAD, head and the connection tab. It wouldn't hurt to grease the thread before re-tightening it.
Regards
SS
November 2007