Thumping coming from duct work

[BIGBREW]

I am getting a thumping in my duct work at the end of a cycle. Anyone have any secrets in how to find the darn thing. It's making me nuts running downstairs every time the furnace cycles through.

 

[HudsonDIY]

Sounds like a damper closing. If that's what it is it's nothing to worry about, it's doing what it's supposed to do.