Thumping sound

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Old 12-29-08, 04:37 AM
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Thumping sound

I hear a thumping sound when my furnace is running. The sound seems to occur about 10 ft away from the furnace. It sounds like a dull metal thump as if you were lightly smacking the ductwork with a rubber mallet.

The sound is originating from an area of the house that is pretty unaccessable (tight crawl space), so I can't get to the source to see what it might be. There is a duct near the where the sound is coming from. The PVC vent is also in the area as well as the copper A/C lines that run to the outdoor condenser.

I have a new Goodman 95% efficient furnace, and just started noticing this noise about a month ago. Any ideas as to what it might be???
 
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Old 12-29-08, 01:46 PM
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The new furnace is probably moving more air than the old furnace. The duct work is probably marginal for the new furnace. It may be the blower drawing air through the return duct is causing an oil can effect. The duct may be flexing and causing the noise.
 
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Old 12-30-08, 04:10 AM
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Would it have to be a return air duct, because there is no return air ducts in the area?
 
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Old 12-30-08, 08:47 AM
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Originally Posted by ideburger View Post
Would it have to be a return air duct, because there is no return air ducts in the area?

Jumping in...

It could be either return or supply ducts depends on your ducting system and how high a static pressure you are running. Does the noise just happen when the circulating blower starts or does it continue throughout blower operation? Are all your warm air registers fully open?
 
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Old 12-30-08, 09:57 AM
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It seems to be the loudest and most frequent when the circulator blower starts up. It slows down as the blower is on for some time. I think all the vents are open, but I will check to make sure when I get home. I believe the duct in the area of the noise is a 6" round galvinized steel duct. I would say it is about 10 to 15 ft in length and originates from the plenum if not within the first foot or so of the main trunk out.
 
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