If the speaker buzzes it is getting something from the amp. Poss bad earths or bad terminations on speaker or amp wires. I have not had radio/audio problems yet........ Can a dealer/indy test the unit on the bench to eliminate problems??
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i