What's that sound?

I'm sitting in my living room, watching TV & knitting a scarf when I hear this constant "whooshing" noise. It ebbs & flows, very quietly & then stops. I look around all confused: what could it be?

*The neighbor downstairs vacuuming? Nah.

*The baseboard heating up in our condo? Nope.

*The dishwasher washing the dishes? Not possible -- it's not running.

Then I turn around & look out the window & see it: THE RAIN! It's pouring outside & the wind is battering the rain against the windows.

THAT is the sound I've been hearing all afternoon? Wow. Mother Nature sure can unleash her fury when she wants to!