is the car also actually stalling? or just a noise? is it only starting from a stand-still it does that, or also if u press the peddle from going a contant speed like 40mph?
it could be a loose battery terminal, make sure they are tight. ask the shop what wires they could have disconnected and 'reconnected'. do u have a 'check-engine' light on? if so maybe a O2 sensor wire didnt get plugged back in. some cars need exhaust parts remove in order to swap a tranny, and so then they have to unplug the O2 sensor wires.
have them drive it and check it out
