Those are the brakes you are hearing.
The PTU is a GROWL that you hear as the engines are started as the PTU runs through a self test regime.
If the aircraft is single engine taxied, then you will hear the growl some more, but that is not too common.
But the banging barks are the brakes.
Aviaco, you probably expienced a single engine taxi if it took 15 minutes from getting off the runway to the gate. In thate case you were hearing the PTU. Short Growls or grinds every few seconds is about right.
Cheers
Wino