I have had on and off for at least 10 years in which it sounds like machinery being used underneath the earth - it goes from constant to only during certain times of year (seasons) - I have researched sites where other people have posted of a similar occurence one woman even was reported to have committed suicide because she could no longer tolerate the noise inside her head. There is a vibration along with the noise. I have gotten so used to it that I just accept it and move on, I have been told by physicians that is goes along with your blood pressure and it is t - I have had hearing tests that have shown no abnormalities= one doctor said it is like the sound of your arteries as blood is pumped through them - it is like hearing your heart beat through a stethescope s= it is quite a mystery but I am at the point of acceptance as just an oddity = a bit comforting to know other people also experience it. My husband used to tease me that I was listening to Middle Earth and its inhabitants. All of nature gives off an electric vibrations and certain people are sensitive to its sounds