Thanks for all the replies

the website on the first message is spot on with the description of the hum
being a low frequency resonating diesel type sound having no directional
source and "heard through the base of the rear cranium

M


wrote in message
oups.com...
In some instances at least, could the hum be simply a next door
neighbour's fridge or central heating system? I sometimes hear sounds
at night which emanate from outside the house, and I always put it down
to that.

Having said that, obviously a hum with no next door neighbours would be
a little strange...

Nick