What causes the furnace's blower motor to fluctuate in speed ?

Answer

Different modes (a/c, heat, fan) will sometimes run at different speeds while some furnaces also have features like 2 stage heating or cooling which will change fan speeds as well as variable speed and modulating units.

Updated on March 23, 2018

Original Article:

How to Quiet a Noisy Furnace
By Dan Robbins
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