I was thinking the same thing, that the motor may be going out, so I tried two other motors, and I get the same results. The faster I run them, the quieter they are, but the slower I go on the speed, the louder the motor whines. In past years I'd seen others complain about the same thing when using a PWM DC Motor Speed Controller, so I just figured it was part of the deal. I was just hoping someone may have come up with a solution for it. I had not tried packing the gears with grease though, so I'll give that a try. Thanks for the suggestion.