The car is a 1996 Passat TDi - i.e. a deisel.
The radio interference is a clicking noise, present in weak FM areas, which is solely dependant on road speed. It speeds up as I go faster.
It continues even if I switch the engine off
The only electric device I can find related to the transmission is the sender for the speedometer. I tried disconnecting this and it made no difference.
I am stumped.
Any ideas?