Welcome Jack, to the best darn DIY"r site on the web.
Is the fan noisy or the air coming out of the vents?
Generally air noise is caused by undersized ducting, slowing down the fan will also reduce the amount of air being circulated, less air may cause Acoil freezing and in the heating season high temp trips.
To slow the fan down you would need to know whether you have a multi-speed motor.
If you do have a multi-speed motor then wires could be changed to run at a lower RPM.
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