I don't remember a flapping sound from the Herald with W/M on, just lots of noise. One or two of our Captains had over 10,000 hours on them and were a bit deaf.
I did 2 years line maintenance with CEX at Hurn on Heralds and used to fly with them around the night freight routes, before becoming an F/E on the Electra.
In an earlier life, I also remember hand pumping de-min water into the 1-11 tank but this was from a small bowser that we towed around in Muscat.