I have a new Lennox 5-ton outside unit, occasionally when the fan motor turns off, there is a loud bang coming from the unit. My service tech is telling me it was caused by damage from the company that replaced my roof, and I need a new unit. Is this caused by a bad motor? Or is it possible small bits of debris have damaged the unit to the point where it needs replacement?

I have a new Lennox 5-ton outside unit, occasionally when the fan motor turns off, there is a loud bang coming from the unit. My service tech is telling me it was caused by damage from the company that replaced my roof, and I need a new unit. Is this caused by a bad motor? Or is it possible small bits of debris have damaged the unit to the point where it needs replacement?

If I were you I would get a second ON SITE inspection of your unit and do not tell the new inspector/contractor what the other one said.

Could be the return is "oil canning ". Under negative pressure when running, it sucks in slightly. When it shuts off, the duct pops back. Had this with a customer and we corrected it by putting a small piece of angle iron across ductwork where noise occurred. No more noise!