Quote from fantom:
"...you missed out the reason why he heard it: the PTU undergoes an automatic self-test after the first engine is started."
Thanks fantom. Assume you are current on type, whereas I'm not, and my FCOM is 1988...Have been SLF for many years now, and have recently wondered about the running of PTU during engine start, as there is normally no demand on the opposite-side system to justify more than a single "grunt". The self-test of PTU on first engine start may have been a sensible retro-fit, so thanks for the info.
Further points for notaplanegeek. The other time for hearing the PTU is during single-engine taxiing after landing, when the opposite-side system drops more than 500psi below the engine-driven one. Have never understood quite why the A320 PTU is so audible in the cabin: other types have similar devices. Most common time to hear the Yellow AC pump is before departure, when the cargo doors are being closed.
rudderrudderrat is right about the shock-wave. It's by no means limited to the A320 and B777, of course. The VC10 used to produce a nice one in the right atmosphere. It migrates back as the Mach No. increases, as you can see from the link. On Concorde, it was behind the aircraft above M1.7, resulting in the drag reducing above that speed (cruise being M2.0).
The Kiwi aviator raises an interesting point about the power-push unit being noisy, but I haven't heard it.
Chris