I took my X5 4.8 in Monday for this reason and went for a drive with the Tech. Of course it didn't happen again but I did confirm that he heard the strange low pitched whistle between gears. They had my car until Wednesday and said they could not duplicate either the BANG which as you said was like being rear ended or the whistle sound. I picked it up and on the way home could hear the whistle sound as before but no BANG yet.