I have just installed a retrofit sat nav system in my E46 M3 with bm54 radio and at low volume levels the sound turns into noise distortion also.Turn up the volume a little and all is fine turn it down and the noise comes back.Does it with radio cds or cassettes. I thought it was the 16:9 screen as the guy i bought the radio module from said it was brand new but now your thread has got me thinking it might be my BM54 tuner also.Not sure how to test as i dont know anyone local with a similar set up.
Anyone any input on this situation please?