We just moved in to this 1992 built house, which has a 10 year old furnace. I constantly hear a knocking ( TAK TAK ) mixed with semi loud banging noise from the vents through out our house. It comes from different air ducts. However when I turn off the furnace altogether, the noise stops after a few minutes.
Its quite irritating in the nights. Please help !

Quite often heat runs are not suspended correctly and they expand and contract. Can you see any of the ductwork, most of the time when I hear of this problem, it is an upflow furnace, located in a basement, and the registers are in the floor. Is this your situation?

Yes, you are right. The registers are on the floor & it is an upflow furnace located in the basement. How could I suspend the heat runs correctly & where should I look for the problematic ones? Is there a difference between registers on the floor vs when they are wall mounted ( like we had in our old house ( 1928) built)? Thanks for your prompt reply.

Look for metal hanger that the pipes rest on. Then simply put a piece of foam insulation between the pipe and the hanger. Sorry I don't have enough time to fully reply to the second part of your question.

Our furnace is in the attic and the ductwork obviously goes downward through the walls. This "tac-tac" noise is in several places going to the first floor of our house. If I remove the drywall should I enlarge the space between the duct and the wood studs or put some type of insulation between the duct and the wood studs?