There should be a lever somewhere on the duct near the furnace. If it is open, the lever will be parallel to the duct. If the damper is closed, it will be perpendicular to the duct.
Damper
If the furnace itself is in the attic, the humming is likely caused by electrical components in the furnace--most likely, the blower motor, or else a transformer or relay coil. If the furnace is located elsewhere, the humming may be caused by a damper motor or possibly the air movement itself vibrating a damper or some sheet metal.The HVAC Veteran
Sounds like there is a damper closed somewhere.
What is open rollout mean on goodman furnace?
Additional details: the damper on the duct next to furnace is marked summer winter. I read the handle should be parallel to the duct in the summer indicating it is open. this duct is not straight but slightly curves. the summer marking is pointed down to the right. is this correct. I have a picture if someone tells me how to post
A top damper or cap damper is a metal spring door placed at the top of the chimney with a long metal chain that allows one to open and close the damper from the fire place.
Shut and Open
what is the purpose of refractory lining in furnace
When you use the fireplace, the damper should be open all the way. If not, you probably will not get enough updraft and your house will fill up with smoke...not a good thing. When it is not in use, the damper should be closed...otherwise you will lose a lot of heat up the chimney.
The open hearth furnace.
On mine if you turn the nob clockwise, that openes the damper. If you trun the now counter clockwise that closes the damper. Yours may be different. Let me think. Did you have anything in your stocking this morning?