Wotcha, AB.
I've been told it's a peculiarity of the airflow across the open gun ports on the P-51D. Can't prove it one way or the other, but it's what I've heard. My observation is that it seems to come out strongly when pulling G, whereas there is no such effect in high speed level flight, makes this sound plausible to me, as only in high G manoeuvres will you get the stagnation point shifting down the leading edge in combination with high energy air flow, making an effect like blowing across the top of a beer bottle. I'll demonstrate with a beer-bottle when I see you, if you buy teh beer..
It sounds from your post as if the low pass to which you refer must have been at low speed, in which case the AoA would again have been high, perhaps creating teh effect even at 1g
Another bit of circumstantial evidence is that I don't remember hearing it at Reno when I attended some years ago, even though I was out at the pylons where they came round at very high speeds and quite a few G. All those Mustangs had the gun ports removed, or at the very least faired off to reduce drag.