Well, if it is intermittent, can you try to isolate it by turning off major appliances, or try to see if something like the fridge is causing this?
I am no electrical engineer, but it sounds like something in your house is periodically running and making this noise.
It may also be something outside, like a neon sign.(-I think there was a thread on this on Agon).
So, I guess what I am saying is that it would be best to try to isolate as many variables in order to get some insight as to what may be causing your problem. When you do have some ideas, then we can offer some solutions.
HTH Bob
I am no electrical engineer, but it sounds like something in your house is periodically running and making this noise.
It may also be something outside, like a neon sign.(-I think there was a thread on this on Agon).
So, I guess what I am saying is that it would be best to try to isolate as many variables in order to get some insight as to what may be causing your problem. When you do have some ideas, then we can offer some solutions.
HTH Bob