Hmmm...
Don't know. It does not seem normal to me.
The PTU works when difference between the hyd pressure is more than 500 psi in either direction, (from memory). It is normally heard on engine shutdown, and as has been said; during a test phase on engine start.
Easy taxy in and out on one engine, so the PTU might be heard then if they don't use the Yellow Electric pump. But on Take off??
Without knowing the answer, I would have to guess that a HYD pump was giving low pressure, or the PTU 500 psi sense was up the bucket.