Living on the south coast, lots of planes pass over Portsmouth heading up to London Airports from Southwestern Europe and Atlantic flights. They would be usually about 10,000' plus. I very frequently hear this peculiar noise. The suggestion it could be a spool up is good, but they can't all be spooling up at the same place. The noise pattern on the ground of a jet engine is very complex, and i think it may possibly be caused by the movement of the noise 'carpet' over the ground, as you pas from one region to another. It is a very strange noise, like a spooling-up growl. It sounds engine related to me.