Thunder from a clear sky?

Weirdest thing, all early this morning I started hearing what I thought was thunder. "Oh great", I thought, "another soggy Sunday". Anyway, I could list of all my bad theories, but it finally turned out to be, of all things, the neighbor's closet!
The apartment whose balcony is level across from mine forgot to push his water-heater closet door all the way closed, and a gust of wind slammed it HARD open into the hand rail. Which is so well attached to the building's frame that it sends a thunder-like boom through the building.

I'm going to go see if I can do something with a broomstick to push it back closed- if he doesn't eventually knock his guardrail out, then I'll go mad from the noise!