Hydraulic power transfer unit - It allows the yellow hydraulic system to power the green hydraulic system and vice versa depending on which has dropped below the pressure that triggers it - cant remember the figure.
If the engines arent running there is an electric pump supplying the yellow system hydraulics and the green system is powered via the PTU.
Its a strange noise for those not used to it and many think its a dog in the hold, barking. Very steadily.....