Very light buzzing noise from the rear

I was coming home from work this evening and pulled into the garage, shut off the car and got out. I notice a very faint buzzing noise...so slight that I thought maybe someone was running the sweeper in the playroom above the garage (like that would ever happen). I finally decided it was coming from the rear of the car, but still was not really sure. On the way home I was jacking with transmission shifting into M1 MD etc just to see what it was like. Not sure if that is related. Anyhow...I started it back up...shut it off and it was gone. I know I've read some transmission issues with 07 X3'3, mine is an 06. Any ideas?

Answered my question by doing a search...seems to be normal! That's why I love these sites...you got questions and someone has answers!