Backfire "Pop" When Engine Shuts Off - Normal?

[Dirt_McGert]

It doesn't exactly "over run" (or "diesel") when the motor shuts off.... when I turn the key to off, the motor stops cranking like normal, then at the end lets out a whisp of air (possibly the exhaust valve opening and letting out the compressed air? - sounds like "pshhhh"), and then a half-second or less later, the muffler lets out a nice little pop.