Why Does Sheet Metal Make Noise

Following are common duct noises and what may be causing the problem.

Why does sheet metal make noise.

The noises are usually caused when the thin sheet metal of the ductwork expands to accommodate the sudden change in pressure caused by the airflow. This is the wobbling sound we hear. Sheet metal contracts when the weather is cold and rapidly expands when the furnace cycles back on which produces stress at the seams of the ducts. As for the popping noise in the morning and evening that s a different mechanism.

To relieve this stress the sheet metal buckles slightly outward making a booming or banging sound. This is influenced by how stretched the sheet is. The actual popping sound comes from the sudden nature of the pan surface as it flexes hitting the oven rack. The metal in the pan expands as it heats and causes the center of the pan bottom to flex making the sound.

This slightly speeds up and slows down the waves moving across the sheet making the pitch of the sounds they are making rise and fall. Duct sheet metal may cause all sorts of noises from one or several different reasons. It s thermal expansion and contraction. There could also be a problem with the fan or air handler that s causing the blower wheel to bang against another part of the unit.

With some experimentation i ve found that a sheet when wobbled gently makes a wub sound which has a different spectrum than rolling thunder. Rattling noises are often the result of loose metal ducts knocking against each other. Quite often these sorts of pops and bangs occur right after a duct cleaning. As ducts age the sealing fails and duct connections wiggle loose from the static pressure.

The sun hits your roof and heats it up. However when wobbled violently the sheet forms bulges which narrow and sharpen and then suddenly flatten making a loud blap like a thunderclap. As the sheet wobbles it is slightly stretched by a small amount. You may also be hearing loud creaking noises from the ducts expanding and contracting from pressure and temperature changes if you have sheet metal ducts.