Originally Posted by
baseball
What's with all the strange noises you hear while taxing around inside the airbus? Never heard them in the cockpit while jump seating, only in the main cabin. It sounds like R2D2 is introducing himself to c3Po in the middle of a electric motor and servo show.
If they are taxiing single-engine, it is the hydraulic transfer pump/motor. It uses pressure from the operating system to drive a hydraulic motor, which then drives a hydraulic pump to pressurize the opposite system. (Been 13 years; apologies to the Bus drivers if the nomenclature isn't right).
But it only runs until the other system comes up to pressure---then it stops. If there is a new demand on that system (flight control movement, for example), the pressure drops, and it happens again.
Same thing happens at the gate when arriving. Right engine gets shut down, and cargo doors (hydraulic, if I recall correctly) are opened. Transfer pump starts running, and everyone thinks it sounds like a barking dog or a big double-pull lumberman's saw. When the left engine shuts down, the transfer system slowly fades out, like the dog is falling asleep.
It seems to be located in the belly between the overwing emergency exits, and that is where it is the loudest.