I have a feeling this might be something to do with the new way I've been using to make MP sound less taxing to people's systems. It really shouldn't make any difference, but it's FlightGear and anything can happen, so I want to look into it and would appreciate any observations people could make about when it happened and how it manifested.
I had it happen to me. The details are a bit fuzzy. It might have been when you were parked at EGOD testing Compositor with VooDoo. And I was flying a chopper. Then you probably left from MP but the engine sound was left behind.
Hrm. If my memory serves, I didn't actually start the engines on the Typhoon when we were playing with Compositor on that Sunday.
And if it's happened when the Typhoon wasn't even around... that makes it a lot weirder!! I thought I had my finger on which sound it was and why it was behaving differently from the other sounds - but if the model isn't even loaded, I have no idea how that can even happen!!
Sorry, I should have added that, if it's the sound I think it is - the phasing sound, which I call the "wake", designed to be heard after the aircraft has past - that might not be associated with the engines running (but obviously should be).
But either way, it ought to pack up once the MP aircraft has quit obviously. I think I have decided on a workaround. Thanks for all the help!