What on earth is that noise?

I'm currently in my office and there's this piercingly dull sound coming from the other side of this wall.  It's a long, consistent, almost blowing type noise.  It invokes thoughts of a ship leaving port or dying moose without the odor.  It's as if a five-ton teapot is finally filled with boiling water.  The noise started during the summer of this year.  Over the past few months, it has become increasingly frequent, typically starting at about five in the morning.  When I was overnight during those summer months, this sound was my alarm clock.  Over these past few weeks though, the sound is more random in its occurrence.  Sometimes for minutes, sometimes for a few seconds.  No matter how it appears, when it appears, it's consistent and its consistently annoying.  The easiest thing would be to get up and look what's on the other side of the wall ... Ahhh screw it.  I'll leave it to my imagination.