Yep, it's likely to be the PTU.
You tend to hear it a lot more now that operators are taxiing around on one engine trying to save fuel.
After pushback sounds like a normal place to hear it (its inhibited during pushback).
Some say it sounds like a dog barking.
If you're wondering what a PTU is, this may help:
A bidirectional power transfer unit enables the yellow hydraulic system to pressurize the green hydraulic system and vice versa.
The power transfer unit comes into action automatically when the differential pressure between the green and the yellow systems is greater than 500 PSI (hence why you hear it more when only one engine is running.)
Hope that helps a bit.