Originally Posted By: rpalac
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Here’s another educated guess in line with what was said.
I have notice that my oil fired hot air heater makes a loud sound when it starts. I investigated it to find that it was the feed air duct plenum expanding (flexing) and making a loud, BANG!
I researched it some more to find that when my air filter which is a 90% is dirty it restricts the air flow enough that the unit builds up enough back pressure to cause this flexing across the sheet metal duct.
My thought was to put in a stiffener with some quick zap screws to alleviate the sound. (A new filter also works until it gets a little dirty). A close friend of mine who is a steam fitter told me this is not uncommon as one of his repairs in the field. He also explained when they bend duct work you usually see a diagonal crease in the form of an "X" in the front and back face of the duct work. That crease is for exactly that reason, to stiffen the duct surface from flexing.
I thought it was interesting and maybe was in line with what you are experiencing. If not, oh, well. It's a piece of info for the future.
(right now I left it so I know when to change my filter.....it acts like an alarm.)