when the furnace fan is running in cool down mode, it sounds as if a flap on the exhaust vent is flapping in the wind

Sounds like you may have a clogged condensate trap, and the inducer housing is flooding. Follow the drain tubes from the inducer and secondary heat exchanger to locate the trap. Remove it and flush with water. This should be done every year prior to heating season, in addition to cleaning the flame sensor (consult HVAC1000 on this forum if you need a pic of sensor).

where is the noise the loudest, and do you hear it clear upstairs?

where is the noise the loudest, and do you hear it clear upstairs?
It gets louder as she gets closer. :D Sorry, couldn't resist...

it is loud at the unit and it sounds like it is coming from inside the unit above the burners.

where is the condensate trap located?

You didn't mention a model #, so...most likely located on the outer lower sidewall of the furnace. Usually black plastic with several hose connections and an outlet piped to a floor drain or pump.

thanks! model GMP100-4

if the trap was that full of water the furnace would probally not fire

Ok, I figured it was a GMPN when you mentioned an exhaust vent. GMP's don't have a condensate trap or secondary heat exchanger. How old is the unit? If you can post the serial # that will tell us.

it is a 2000

i will look for the serial #

thanks

the model #U-36

serial #0009594323