I have a Dial A Temp speed controller on my Buck 91 to slow the fan (Mostly for sound) and now i'm getting a sound from the motor that sounds like a bad bearing. If I run it fast for a moment (20 sec) it will subside for a while (30 min) Last night I took the motor out and checked/Oiled and it seems fine. if I take the Dial A Temp off and run it on low it's so loud that I cant hear the noise. Can these shorten the motor life?? Any Ides?? Thanks