The audio wiring in your adapter harness may not be correct. Try the fixes discussed in these two threads.

#1: MKIV Voice Buzz Issue SOLVED !!
#2: Nav Audio - Success

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input