Yes, it is the PTU.
I am no longer 100% sure how it works but if memory serves me right, this is how it is:
During start-up of the second engine (Engine 1 when it is a normal automatic start), the PTU makes a self-test that lasts about 10 seconds IIRC.
After engine shutdown the PTU is de-energized which makes the "barking" noise slow down because both green and yellow systems slowly depressurize.
When you activate the Yellow Electric Pump from the overhead panel (or when a cargo door is opened from down under), the sound of the pump is actually a loud and high-pitched noise.
You cannot hear both noises from the cockpit, but you'll most likely hear them in the cabin, especially if your sitting near the overwing emergency exits.
When dead-heading in the passenger cabin, we would sometimes tell curious passengers (as a joke of course) that there was carpenter in the cargo compartment opening the cargo doors with a saw.
Hope this helps.