>>3732201Its been determent to be coming from the ground and resonating in the air.
The core of the earth is partly molten lava, there is a iron ball the size of Germany rolling around in the lava core. When it passes near your area you will hear the resonance. (trumpets).
This is due to the 2,000 / 3,000 year pol reversal.
link if you'd like to learn a little something.
http://volcano.oregonstate.edu/book/export/html/1098 