Hi thanks again for the replies. Re the ear plugs, we already wear them and the really good ones as well - the noise goes right through them. My electrician said that if the problem is electrical, the electrical waves will go right through any ear plugs, but if the problem was any other sound, it wouldnt.
Thanks to asking one of my neighbours,(who came into the house and had a good listen) I think I have finally figured out the problem- we have cables running right outside each bedroom window. I got on a ladder and there is a distinctly loud hum coming from them. When I go back into each bedroom and go to the wall where the cable is on the outside, the humming is louder there. So it sounds as if the noise of the cables is coming through the walls. As we are there three years now, I cant figure out why the cables should be only vibrating so much more now.
While I have the ESB coming out on Tuesday, my problem now really is what they will do about it. What bugs me a bit is that my own house seems to have quite a number of cables around my windows, whereas the neighbour who helped me, his own house has just one cable running down it.
I will let you if the ESB agree that its the cables!