Also worth pointing out that this normally only occurs when one engine has been shut down during taxi in after landing. The PTU only runs when there is differential pressure between the two systems and during the single engine taxi (always No.2 shutdown) the electric pump on the yellow system is switched on to keep the system pressurised which means the PTU won't run. It is done to prevent wear on the PTU which is considered a critically important system, so important it is automatically tested at every departure during engine start. Once the aircraft is on stand the yellow pump is switched off and the No1 engine shutdown leaving differential pressure across the green and yellow system which runs the PTU while No1 engine pump runs down.
At a previous airline I used to just reach up and switch the PTU off during the shut down scan to stop the noise in the cabin, it is rather frowned upon at my current airline so now my passengers get the barking dog.