Why does my gas furnace clank right before blowing the air when it comes on?

Good question and sorry to say I cannot hear it from such a long distance.

I would check the blower assembly for the furnace.

Bad rubber mounts,bad motor belly mount or welded arm assembly (depending upon the style you have) can cause this type of problem. A loose cage setting screw can also allow the cage to slip on the shaft and due to play in the shaft can cause a scrape type sound.

Best to pull the assembly and perform a inspection. While you have it out you might as well disassemble and clean. I am sure the cage blades are dirty.

Are you sure it is right before the blower comes on, or is it right after. Is it a new problem or has it always done it. It may just be your ductwork popping. Does it also make the noise when your blower turns off? If its not your ductwork, then follow hvac1000's instructions. Ductwork popping will usually sound more like a drum.

Good question and sorry to say I cannot hear it from such a long distance.

I would check the blower assembly for the furnace.

Bad rubber mounts,bad motor belly mount or welded arm assembly (depending upon the style you have) can cause this type of problem. A loose cage setting screw can also allow the cage to slip on the shaft and due to play in the shaft can cause a scrape type sound.

Best to pull the assembly and perform a inspection. While you have it out you might as well disassemble and clean. I am sure the cage blades are dirty.

This didn't start happening until I had the furnace cleaned and inspected in early spring. It is not the ductwork popping. It is loud clanking and squeaking and it never did this before.

are you sure it is right before the blower comes on, or is it right after. is it a new problem or has it always done it. It may just be your ductwork popping. Does it also make the noise when your blower turns off? If its not your ductwork, then follow hvac1000's instructions. Ductwork popping will usually sound more like a drum.

Thanks for the info. It is not the ductwork popping. It just started after having it cleaned in early spring. It does not make a noise when it shuts off.