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Can be a few things. Fan inducer motor may have failed. Or blower that moves the air may have become loose or is out of balance. Or it can be from what is known as Oil canning.
I suspect that its oil canning which is why I am talking about this first.
Oil canning is when the return air duct gets sucked in when the blower turns on causing a loud banging noise as the metal flexes. Then when the furnace is turned off it bangs again when it returns to normal. This can happen if there is a remote filter in a wall or ceiling elsewhere that may be plugged with dirt, Or a blocked return grill somewhere else in the house. You can check to see if this is the noise by having someone turn on the furnace while you watch the air ducts that go into the bottom of the furnace. If they move in and out when the system turns on and off this may be the reason for the noise.
If the furnace is vibrating and making banging noise when fan runs, it may be caused by something that got stuck in the fan blades. Perhaps the filter may have become dislodge and part of it is in the blower.
Just a little piece of paper in the blower can make it go out of balance quite easily and make a racket.
If you suspect that this may be the cause you need to do the following.
Turn off the furnace at the safety switch. Then remove the bottom panel on the furnace to expose the blower fan. Then carefully reach around the side of the blower that the filter is located on and feel the blower blades with your fingers. Be careful as they can be sharp. But feel around as you slowly turn the fan with your hand. You may find a small piece of paper or part of the filter trapped in the blower cage. Remove it and put cover back on furnace bottom and turn it on to see if this noise has stopped.
The fan inducer motor can make noise as well, but quite often then not it fails and the furnace does not even run.
Its common for people to use the electrostatic type filters thinking they are making the air
Gas furnace: Up draft, down draft, wall heater, efficiency heater, convention natural gas.
When it calls for heat, do you hear the IBM (little blower motor) taking out the unwanted gases first. Then you should hear a click releasing for the circuit to go through the safeties. Then some sort of ignition source, then a click to open the gas valve, finally lighting the gas. In two
If it's not doing this in sequence, you could have a bad circuit, circuit board, ligniter, gas valve, or blower motor. My best guess is that your blower motor is going bad if it's a loud ____________?
Anyway. Great response. U know your stuff.