My answer to What is that strange pumping/scraping sound coming from underneath an A319 when taxiing?
Answer by Thomas Zerbarini:
That sounds like the Power Transfer Unit (PTU) on the Airbus. The PTU ensures adequate hydraulic pressure across the green and yellow systems during single engine taxi operations. That is why you here it after pushback (one engine started) until you get to the runway (second engine started).
Here is a picture of the Hydraulic panel:
Here is basic schematic display of the Hydraulics showing the PTU in the center linked to both green and yellow systems and capable of providing cross-side pressure.
You’ll even get that barking dog sound sometimes when the PTU is operating. check it out on YouTube: