Very similar experience, after a bit of research it was clear the 'global hum' explanation loses out to the much simpler explanation that it's an internal effect.

I too found exposure to loud noise supressed it (e.g. regularly suppressed for a couple of days after nightclubbing), also only 'audible' in very quiet surroundings (even lightly touching an earlobe caused the 'hum' to cut out).

It went away after life got noisier (kids'n'dogs).