What's that noise?

[Espada III]

On the many flights I have taken on Airbus planes (usually A320s or similar) there is a moderately high pitched 'winding' noise that comes on as we start to move away from the gate and after we park up.

It sounds a bit like an electric screwdriver but more painful.

Any ideas?

[22/04]

Think it's a power transfer unit, basically pressurising the hydraulics. There have been other threads but I can't find them right now.

[Avitor]

Would it come from flap setting?

[PAXboy]

Probably the Power Transfer Unit.
http://www.pprune.org/passengers-slf...ound-a320.html

[Greek God]

AB has 3 Hydraulic systems Blue Green and Yellow. Blue and yellow are connected by way of a power transfer unit (PTU) this allows the one hyd system to drive a pump pressurising the other & visa versa, they are normally powered by engine driven pumps. On pushback starting No2 (right engine) will pressurise the yellow system which will start a ptu test and power the blue system. It the PTU running which sounds like dogs barking or a sawing sound. The reverse happens on taxi in if the right engine is shut down unless the yellow electric pump is turned on.
Hope that helps but it is quite normal