What can make a transmission "BURP"?

[Rrumbler]

Not a big thing I hope, not my car, didn't seem to cause any problems, but it's got my mind spinning this way and that.

Wednesday, I took my son's BMW 5(?)6 to get it washed, and took it for a ride, since it had been sitting for a few weeks.  Now this is one of those super whizbang cars with more brains and go than me, turbocharged, six gear automatic tranny, and more electronic crap in it than I can sort out without a thick book, but I can steer it and make it go and stop. What I can't do is crawl around under the hood - can't tell much from just looking anyway - or get down and look under it without needing a hydraulic jack to get me back up off the ground.

After the bath, I took it for a ride out north for about fifteen miles and back to dry it off and give it a bit of a workout; it ran like it always does, smooth, fast, and quiet. On the way back home, I stopped a the drive-through pharmacy, and while sitting, waiting for the lady to do her stuff, the car started smoking; seemed to be coming from underneath and towards the rear right side.  I was in that driveway next to the building and couldn't get out to look, so I just sat there and hoped it wouldn't grenade on me.  After a minute or so, it stopped, then started up again a few seconds later.  As I was pulling out, it stopped smoking completely, and didn't do it again on the way home.  There were no alarms, warning lights, or anything to indicate a problem other than the smoke; it had a smell sort of like engine oil, but not as strong, definitely chemical in nature, though.  It did not smell like coolant that I could tell, at least not like any I have ever smelled; there were no drips or spots in the driveway, and when I got home and parked it in their drive, I looked under it as best I could and saw no drips or spotting.  I'm baffled.

They are home, off the road for a week or so, so I imagine they will get it sorted, but it is digging at my head.  I have heard of automatic transmissions doing this, but never experienced it.

What do you folks think I ran into here?

[idrivejunk]

I vote that a small pool of leak from sitting rolled off the belly shielding that BMW is so fond of. Then said fluid found the exhaust about when you turned in to the store. Might be cause to check fluid levels, but what you described wouldn't take much.

[Rrumbler]

Thanks, Matt, it appears that that may be the case.  After considering the whole event, we think that the car wash undercarriage wash left a lot of water and cleaning solution trapped in places where it eventually found its way out and onto the hot exhaust during the trip out and back, and when I stooped at the pharmacy, it was still steaming off and sitting still, it was noticeable, as you said.  The car is pretty clean underneath, no signs of oil leaks.