Audio Systems: Static, bmw car, car radio

Question
Our BMW car radio is full of static when on our local AM station. Strong FM stations work ok. Previous cars and other cars do not have this problem with our local station.  Dealer says cannot find problem .  Can you help.
Thanks  

Answer
first, i need to ask a few things; does the static vary with engine speed? does it stop completely with engine off? is it gasoline or diesel? please describe as best as you can, the sound, is there a 'whine' with it? i just need to try to narrow it down a bit. also, have you had either the plugs or plug wires changed recently?