Model #U-36 serial #0009594323

The furnace is making a loud clanking noise when shutting down/cooling down.

Please advise

Kill the power to the unit and remove the blower access door. Then reach in and see how much up and down/ in and out play the motor shaft has. It is possible that when the motor comes off speed the shaft moves and allows the cage blower to rub the sides of the blower housing.

Could be the ductwork poping back in or out after the fan shuts off

hvac1000

I checked the play in the blower motor shaft and there in no play. Pretty solid. The sound seems to be coming from above the flame manifold. It sounds like a flapping metal door in the wind. Is there some sort of vacuum mechanism that would hold the external vent flap closed? There is a black vacuum line, but I am not sure what it does.

Thanks for the help.

The model number you posted above (U-36) is actually the A/C coil part number. Post the real furnace model number which should be on a tag inside the furnace burner area door. It will contain other information also but the complete model number is what I am looking for. I will see if I can look up your furnace model number and find something that could be causing the noise.

Is this a strait HVAC system with no zone dampers etc?

Model GMP100-4
Serial #0011628732

Thanks for the help!!

Sorry

There is no damper system. Straight system

The manual listed here is for service and repair of a few different models and yours is listed. I sugest you save a copy on your computer.
I have looked over the manual and the exploded diagram of your unit and see nothing that would leed me to believe would make the sound you mentioned.

The only thing I can think of would be a loose metal strap that is used on some units to support the A/C coil but that would be a long shot. It looks like you are going to have to hunt for the problem. Remember you are dealing with a metal box so to speak along with the connected duct work so the noise could be coming from almost anywhere. Joshdta mentioned the suct work and that could also be the problem.

http://www.hvacpartsshop.com/Gmph075-4%20Series.Pdf

Thanks for your help

I hope you find the noise.