Client states loud banging in ductwork?

I had a customer e-mail me about a loud constant noise(banging)coming from his ductwork over the furnace and in the master bed and bath. It’s only when the fan is on and has become worse since the utility company replaced the fan motor. The noise stops when the fan shuts off! IMO it sounds like a imbalance/restriced air flow or loose ductwork that may need strapping. Any ideas would be helpful or maybe someone has has this situation happen before.

Thanks in advance!

Does he have rectangular metal duct or plenums?

Does it bang as the furnace starts or after the duct warms. they some times don’t get the break in the metal for stiffing and the metal flexes.

Maybe a damper.
Never knew utility companies replace furnace parts.

Terry’s right, that’s why I was asking if it was metal duct as opposed to fiberglass ductboard. Metal should have seams like this to stiffen the sidewalls. If not, it will pop & bang.

1 Check Anything blocking the return Air
2 check if motor mounts have not come loose or it is not the right speed for the furnace and it is running the right direction ( many fan Motors come with both directions)
3 Check dampers to make sure they have not come loose and are flopping around
4 Check Fan door make sure it is secure
5 Also check if they can there is not something laying in the duct

Thanks for the input… It’s been 8 months since I was there and it’s a slab home so I did not see most of the duct-work or was this problem present when we ran the A/C back in May. I do believe it’s a metal plenum coming off the top of the furnace, but after that it’s concealed in the floors! The banging or noise is only heard above the furnace and in the master bedroom and bath(above utility room). I mentioned most of your comments before I posted, but haven’t heard back. It does only happen when the furnace comes on and until it shuts off. Any other ideas?.. I told him to call a HVAC tech to further investigate.


The banging sound you hear is caused by thermal expansion and contraction of the metal when either warm air flows through cold ducts or when AC runs through warm ducts in the summer.