Surely it's simply ducting when a strong inversion (as we had recently
until today) traps the noise?

I am about 4 kms from the busy A505. I never hear it unless the is an
inversion. Equally, aircraft flying above the inversion are heard less
in these circumstances.

Presumably a "hum" is a combination of various, normally undetected,
noises? It certainly must be annoying, but the reason seems fairly
obvious.

Next time you get a cool unstable Polar airmass, I bet you don't hear
the noise as badly.

Jack