This seems to be an old thread, but here goes with a new theory. I have been experiencing the low frequency noise with vibrations for about four years. I have tried most of the items mentioned to try to find the cause. When none of the "solutions" panned out, I hit the internet and scoured around for possible solutions. Here's what I am currently thinking:
I live in a (mostly) quiet suburban neighborhood. My neighbor of 30 years is a retired, do-it-yourself person who lives on minimum income. My latest thinking is that he has purchased the necessary materials and build himself a magnetic motor which which to generate his own electric power. Check out this website:
DIY Magnetic Motor | fuel-less motor | money saver
As far as I can tell this is a form of perpetual motion machine which would account for it running at some level 24 hrs. a day; the fact that the hum seems to be "electrical" in nature; hum/vibration varies in intensity as if on some sort of rheostat.
The vibration I feel from the noise can be felt by me all through my house (also 3 neighbors houses), but I do not feel/hear it any where else. At times it has been so intensely annoying that I have had to leave my home. Also, my husband does not hear/feel it.
Anybody have any experience with this "magnetic motor?"