it possible is the blower motor is going bad or the blower motor squirl cage is rubbing
Most likely there is debri in the blower motor cage, usually mice get in there.
It serves to release abnormally high pressures before they damage a system. For example, a water heater contains a pressure relief valve. If a thermostat malfunctions, the heater would continue to heat, turning water to steam. The higher pressure created by that would cause the water heater to explode- but the pressure relief valve reacts to the higher pressure, and vents it off safely.
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