The whistling noise could be either a problem with the noise cancellation circuit in the headset (this uses a feedback loop and can thus go unstable)
That's exactly my suspicion.
or it could be due to the aircraft intercom not liking the impedance of the headset.
It would be very surprising if the input impedance of the headphone changed significantly when the active circuit is turned on. So I think this is more unlikely.
Does the headset have a battery, or does it draw power for the active circuit from the intercom? If the latter, then that could cause it to be dependent on the combination of headset and intercom: if the intercom isn't supplying quite enough welly, the active circuit might malfunction and go unstable. Or it might even be an oscillation on the power supply from the intercom - rather unlikely, though.
Mad Girl: Well done on getting into the sky (better than I've managed recently!). And glad you've found a fix. The rapid switch off/on probably doesn't work, because the circuit stores a bit of residual charge and needs to be switched off for a couple of seconds to actually power down, and restore the circuit to its starting condition.
I can only speculate at the cause, but my best guess is this...
[geek]
There appears to be a fault in the design or the manufacture of the active noise reduction circuit, which causes it to go unstable after a sustained period of time in the presence of normal cockpit noise levels and usage. This instability manifests itself as an oscillation high in the audible frequency range (probably around 10-18 kHz from the description), gradually increasing in amplitude, and taking 60-90 minutes to become audible. Powering-down and re-powering the ANC circuit clears the fault. It might be dependent on the type of intercom, though this is less likely as the problem occurred in a number of different aircraft.
[/geek]
Also.....As I got 20-30 minutes more time before the squeal this time - could electronic circuits need to be run in?? i.e. warmed up a few times.??
It's very unlikely that it needs to be "run in" (regardless of what audiophile loonies say about their hi-fis). But the development of the squeal is probably dependent on various factors, like the sound level in the aircraft, or the temperature, or any number of other random things. So it's not surprising that it might take longer one day than another.
It'll be interesting to find the cause, if Pilot set their engineer on the case! Happy squeal-free flying.
mtw