4" Elicent inline-fan + Speedster Fan Control = Motor Hum. Any Solutions?

I bought a 4" Elicent inline-fan + Speedster Fan Control. The motor on its own is silent when plugged in. The only noise is the air being moved. When plugged into the Speedster, the motor hums louder than the air moving through the fan.

Has anyone found a solution for controlling the speed of an Elicent Fan w/o causing the motor to hum?

Putting it in a box, installing a muffler, etc.. is not a solution but a band aid for a problem that will shorten the life of the fan. I am not looking to shorten the life of an expensive investment.
 
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