chasing my nephews and reading leather-bound books. Basically I'm Scrooge McDuck, minus the lisp and bifocals. Okay, okay - there's no vault. Or mansion. And I usually wear pants. I have a good career, and active passport and plenty of friends to fill my less-than-echoey apartment...
If you only hear this noise once in a while,usually shortly after the AC kicks on in the first place, it might be a simple matter of the duct expanding due to temperature. This is especially true if the noise sounds metallic and echoey, like someone tapping on a big piece of sheet me...