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How loud and how long does the screeching last?
If you remove the panel on the furnace and run it does it make the same noise?
Can you remove the panel on the furnace to see if that continues to make noise?
Yes can you just tape the panel switch down and run it like that to see if that stops screeching?
Yes there is a switch that the panel engages when put back in place. That switch should be right up in the corner on the frame edge you should be able to see it.
This may be a whistling noise from the suction through the ductwork at start up, it will not do that if you have the panel off and if it does not then there is a suction issue through the return vents.