duct knocking

[setterguy]

I've got some knocking coming from the ducts when the furnace kicks in...I've eliminated any furnace issues and there is heat so it isn't trying to fire. I've read that the duct work will expand when heated in cold weather which makes sense but here is the question. Is there anything that can be done to eliminate this? It does wake me up sometimes, I'm not afraid to call an expert if thats what it will take but don't want to call one if it won't help either. Thanks.

[Surface Tension]

You have it right, Expansion and Contraction.

Go done stairs and wait for it to happen, usually a while after the furnace kicks in.

Consider the bang like this. As the metal expands it is being held back, when it expands to a certain point its let loose all at once. That could be a duct in a place where it has a tight fit and the metal is forced to warp. Anyway it'll take some time to pinpoint where the sound is coming from.

The fix is a trial and error deal. You'll know your on the right track when the sound becomes muffled.

Could mean creating more space between the duct and framing. Tightening or even loosening joints. Expansion joint(s), or something as simple as a rag between the duct and framing.

A constant banging is the movement of air. You'll be able to see or feel that. That is one reason to put diagonal breaks{X) in long ducts to give it rigidity.