Google it on YouTube and then listen.
I have no clue.
But it does seem that something is up. I suspected a hoax, but it's pretty darn worldwide. Normal sound diminishes at the square of the distance to the source. This doesn't seem to. One popular theory is it's HAARP (Google this.) Another is its the shifting of the earth's magnetic core (how this makes noise eludes me.)
Some think it's a religious thing.
Your theories are welcome.