Transmission failsafe + code P0377

[wellingtonamaral]

So, while driving my '04 X5 3.0i, E53 (GM automatic transmission), it went into transmission failsafe, and I read error code P0733.

Ref: http://engine-codes.com/p0733_bmw.html

Starting the troubleshooting from the cheap possibilities: I did a partial transmission fluid change (without pan drop) and also changed the battery. I was overdue for both anyway.

I just recently bought the car, and it has driven fine and shifted fine until this failsafe kicked in. I can still drive it in manual mode without triggering the failsafe, although the 2-3 gear transition is still a bit slow on the shift.

Relevant or not: I also don't know whether it is normal for this model, but in the first gears, as it shifts up, I hear a loud air-blowing sound...sounds like a turbo -- though there is no such in this car.

Any ideas before I surrender to a local dealership would be greatly appreciated!!
-W.

P.S. mistake in the post title: should read P0733, not P0377. (Thanks RRPhil)