Two penn'orth - depends if they are climbing or cruising. Or upper-level winds, I'd guess.
Having grown up under the old "Amber One" near Castle Don., now living under "some airway or other, roughly OTR-NEW", going "home" makes me realise how different the sounds are.
In North Yorkshire, west-bound transatlantic stuff is generally in the cruise. The only things that make a distinctive racket are Qatari and Etihad climbing out of Edinburgh for The Gulf. Occasionally Emirates out of Glasgow, too.
Back in the East Midlands, the noise of upper airways traffic climbing is quite distinctive and noticeable.
Even further back in the 1970s, the -100 series 747s were very noisy, particularly Pan Am West Coast services, low-pressure fan drone being very clear.
Last edited by Midland 331; 3rd Jan 2018 at 12:56.