Originally Posted by tojo_m
Actually both the times I heard the sound, the car was cold.
Will keep a look out for this pattern.
You're in Chandler, so your car is never really 'cold'. Do you garage your car? Even if you don't, try just letting it idle while the computer boots up, find some good tunes, etc. before driving off.
I know with mine (335i w/ MPPK same as yours), it takes about 30-45 sec on a hot day for the exhaust valve to close and the RPM to drop from 1100 to 500 on cold start up. Try letting this happen before driving off, and see if you still hear it.