Volvo has two TSB's out on this problem in 7xx machines. They essentially recommend: (1) installing an isolator in the back antenna connector and a ferrite core near the radio and (2) grounding the antenna housing and the radio chassis more effectively. Parts are inexpensive and available from dealers. They also write about checking various electrical components (cap, rotor, plug wires, etc.) to make sure they are functioning and reduce noise. I solved my problem with (1) and (2) above and also with a noise reducing unit from Radio Shack at the power lead on the radio. I suspect that more noise could be taken out of the AM spectrum by replacing alternator brushes, etc.