This might help you problem.

Found it here: http://www.bimmernav.com/bmw_mp3_cd_changer.html

Service Measure B65-209-04

"Subject: E39, E46, E53 with BM53 Radio Navigation system, and DSP or Harman/Kardon Amplifier: Radio is switching from Stereo to Mono"

"Complaint: In FM mode only the radio sound is switching from Stereo to Mono, fading in and out.

"Cause: Software error in the BM53 Radio".

"Measure: On a customer complaint basis only, replace the BM53 radio. Install a BM53 radio with the new software"

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