Simple mentions of the Doppler Effect do not explain the noises. Where I am in Portsmouth, I get a lot of London bound jets descending and at about 10,000'+. As they pass overhead, there is a 'spool up' sound' then the noise appears to resume something similar to its previous sound. Doppler explanations do not cover the few seconds of this previous noise. It happens again and again, so I am inclined to think that if you were to draw a frequency 'map' of the sound below jets with this sort of power set, it would show some funny effects immediately below the aeroplane.