heat duct expansion

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I assume you are hearing this sound from heat registers. The sound results from expansion and contraction of metal parts when temperature changes occur. You can minimize the sound by isolating where the clicking occurs and limiting friction between the pieces. Most likely at strap hangers, places where the duct rubs against the lumber frame of the house, and at the register connections where the duct is rigidly held against the floor. This is an installation problem and is not easily fixed.

The trick is to isolate as much as possible the pipes from actually touching any wood framing. The large trunk heating duct is often suspended from the bottom of the floor joists one inch to make sure it does not touch the wood framing. If it is in contact with the joists, it will make noise. The solution is to loosen hangers to allow the duct to move away from the joist. Expansion joint collars can be used to separate the large trunk heating duct from the furnace plenum. These collars absorb much of the expansion and contraction that happens just above the furnace heat exchanger. Short runs of flexible duct connections to registers can also allow the system to move freely without clicking.

Try to find the source of the noise and deal with those spots first to minimize alteration and disruption of the ducts. The best way to find the noise is to cycle the furnace and listen for where the noise is comming from. Deal with one spot at a time as you find noises, and eventually you will solve the problem.