Stolen from another thread, post by I-2021.......
On the A320, the "barking" sound you hear is typical of a situation where the PTU runs to pressurize a hydraulic reservoir with a low hydraulic output. What happens is that it takes only a few seconds to power the reservoir up to 3000 psi and then the PTU stops. In another few second the pressure drops... and at 2500 psi the PTU kicks in again to bring the pressure back to 3000 psi and so on... here you get this barking sound. If you want to hear another sound from the PTU, try pressurizing a reservoir, activating the PTU and using some flight controls, such as flap/slats, you will hear a costant sound from the PTU and not the barking sound you are used to.
Disregard "reservoir", insert "system".
TP