Answer:
Metal ducts make all sorts of noises. In some cases, excessive air flow can cause whistling. Sometimes, pressure differences between the supply and return ducts can move their walls in and out, resulting in a rumbling sound called “oil-canning.” And ductwork can carry the sound of a noisy blower all over the house. But I suspect your problem is caused by hot air rushing into a cold duct, which makes the metal expand. You might hear some innocuous creaking, but if the duct is rubbing against wood, it might suddenly “pop” and reverberate, then pop again as the metal cools. What can you do? Short of covering all your ducts with fiberglass insulation, or opening them up and inserting acoustic duct liners, the best you can hope for is a modest reduction in the noise. Use rubber pads to isolate ducts from wood. Make sure there’s nothing blocking the air register at the return, which can lead to pressure problems. Have an HVAC technician check the system to make sure it’s not pushing more air than it was designed for. And if your furnace doesn’t already have a flexible transition to isolate furnace noises, ask the technician to install one.
from Nov 2005
