There have been numerous post over the last year or so including an origional from me about this noise. I bet you only hear it on the 757's with the PW 2000 engines. Some examples of U.S. airlines that fly the PW2000 are: Delta, United, North West, and the old ex TWA. Some inside people from the airline mechanics industry answered these post but they have since been arcived or deleted. I am not a mechanic, so I will not go into detail, but the noise is the 14th stage airbleed that opens to release air to prevent compressor backfires during rapid deceleration. The air is coming straight out of the core of the engine, so it is very hot and that causes a volitile reaction when it hits the cool outside air. I love the 757, love to fly on it, especially the PW 2000 ones because of that grand finish they make at the end. "And the trombone section stood up!", quote taken from a very good airliners magazine a while ago.