I did not verify the Class D theory...only thought of it after the radio-end install. After further driving I still can hear some noise on very quiet passages with music like classical guitar but it is a lot less than before. I think that this is also load dependent. Seems to happen more often with headlights are on. The real problem is...these BMWs are just too quiet! In my other car I can't here any alternator noise.. it's drown out by the supercharger whistle and exhaust note