Whistle in Holden Statesman WH2 V8

What might cause high-pitched whistle in WH2 Statesman V8? Pitch changes with accelerator pressure, even before engine revs change. Seems to be vacuum-related but mechanics say "no"

All engine pulleys checked and/or replaced.
PCV hose and connections checked OK.
Noise sometimes seems releated to whether the aircon is ON or OFF. Could alternator electrical noise get directly into speaker wires, even when audio system is OFF?