I think 747Focal is most probably right. I live in north Portsmouth and it repeatedly seems to happen in the same area of sky. I descend in this area myself and there is no reason for a particular event to take place in this location. But the sound change is large in frequency- sometimes it seems to be more than just bleeds. I think maybe we need to go into Noise 301 for the explanation- I'm inclined to think the noise footprint over the ground has a frequency spectrum along it that isn't explained by Doppler effects.