Mine is a simple DC motor, so lowering the voltage should decrease the speed. Now I have to ask, did you release the little lock down tabs on the flap outside before toy ran the fan? Makes a world of difference. Withe flap locked closed, the fan has no resistance, and will scream. If you open the little tabs, and let the flap open, then the fan will move some air, and is quieter, plus that is the only way it works.
DO lock it back down before towing, or you will be buying a new flap.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT