Clanging ducts in an older home

[Orrex]

Our house is brick and between 80 and 90 years old. The current furnace is about seven years old, and much of the ductwork was probably put in in the last decade. In the basement, some of the cold air returns are little more than sheet metal nailed to adjacent joists under the first floor--they work just fine in this capacity, but they sometimes clang as we walk around the living room or kitchen.

I'm guessing that it has something to do with the old joists flexing and thereby causing the sheet metal to buckle slightly--does that sound right? If that's the problem, is there a simple solution?

Any advice is greatly appreciated!

[Stinky The Clown]

1. First of all, that the usual way return air ducts are done in frame houses

I think your analysis as to the reason for the 'clanging' is right on the money; flexing joist causing an 'oil can' effect in the sheet metal. It seems to me there are two possible remedies. One is to remove and reinstall the sheet metal such that it has no kinks or 'bellies' in it. If you find the exact spot where it is making the noise, you could also try the somewhat brutish but very expedient measure of slapping the metal with a hammer to dent it. The dent will, in effect, shrink the metal and eliminate the bow that might be the source of the noise,

The other fix is more involved. If the joists are undersized and they flex sufficiently to cause an otherwise correct return air duct installation to make noise, the joist are somewhat undersized. The solution here is to 'sister' another 'two-by' board to the side of the existing duct to increase the joist's stiffness and resistance to flexing. You could do this just along the reutrn air duct .... or you may want to get an engineer to have a look and give you his assessment of the integrity of the entire joist system.

[Orrex]

2. Wow! Thanks for the quick reply!

It's a little late in the evening to try the hammer/dent method tonight, but I'll do a closer inspection to see if I can isolate the precise points of clang. So far it's been mostly a matter of "when I step here..."

I'll give it a look tomorrow and report back here with the results.

Thanks again!