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I live in Birmingham in England and we have the same problem here.
I have heard the on/off hum in three different homes in this area and also in a house in Gloucestershire.
Only inside never outside not even at 05.00 when its dead quiet.
There are no overhead power lines near any of the 4 properties.
It's even been in a local newspaper with other people experiencing the same hum.

Are there wind turbines in the area of the homes?

Tick

Is this article related to your area?

BBC NEWS | UK | Have you heard 'the Hum'? (http://news.bbc.co.uk/2/hi/uk_news/8056284.stm)

Note they mention Bristol, Cheshire, Cornwall, Gloucestershire, London, Shropshire, Suffolk and Wiltshire.

The humming could be the wind or teen boys always going round the house humming