Yes, I'd guess it's caused by the PCUs. IIRC, as has already been mentioned, it was best to apply power smoothly and firmly. If you stopped halfway then the PCUs would have a little yank up and down.
411A,
Military Argosy on approach - reached Water Meth in the checklist - FE switched all four ON with RPM above WM cut-in RPM (don't ask) - 'exercise the PCUs' wasn't in it! - very invigorating
'fine engine': radial compressor certainly ate birds - like a few pilots I've met
ABUKABOY
Bring back the Dak! - Didn't these guys go on! (If that refers to those I remember from the 70s)