OP again - I may have found the answer but its not the one I wanted. A very nice neighbour came in and put his ear to everything, internal wall, external walls, the windows.He could kind of only hear the humming with the walls but when he put his ear to the windows, he could hear exactly what I am hearing. A new motorway opened up only about two weeks ago, neighbour thinks all my windows are vibrating from the sounds of the motorway (its about a mile away) and reflecting it back into the house and its walls. Is he right? Certainly this morning. Its just crazy.When I open a window, I can hear a very slight hum in the distance from the motorway. When I close the window, the sound is much stronger.
I am seriously worried about the insulation, mostly because I am getting such confusing information on it. SEI said that the foam should actually be muffling sound, not causing it, that cables are protected and that the foam shouldnt have disturbed them. Electrician I had in, has foam himself and said the stuff used is supposed to not affect the cables. I did contact the insulation company and they said they never heard of the foam affecting cables (company was recommended to me by a friend who is related to one of the owners, and they are aware that that is how I found out about them so they may not be fobbing me off, I dont know.)
NTL is only happening after Easter so that is still a possibilty and the alarm.
There is one thing - the transformer for the door bell is buzzing. But with the mains off, the humming sound is still there. Im just wondering though if the transformer could still be causing an effect or is this not possible, given the humming is there, when mains is off.
If my neighbour is correct about the windows and the motorway, would getting new windows in (overdue to be done anway) help?
Edited to say I forgot to add one thing - neighbour said there is a mast in a playing field not far from my house, said it could be a mobile phone one (ESB said none are near me). Should I investigate this?