You have gotten something wired wrong at the supply for the radio, probably crossconnected something so that you have a feedback circuit.
Check the wiring around the radio -- it is possible that you got a wire between the radio and bracket and shorted it or something, or spliced something in improperly.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!