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I believe that a DCC decoder controls the speed of the motor by using PWM (Pulse Width Modulation). PWM basically turns the power on and off to the motor several thousand times per second. When you tell the loco to go faster, is just changes the ratio of on:off time so that the motor is on power a larger percent of the time. Each time that the loco gets a bit of power, for that fraction of a second, the motor will accelerate. It will then decelerate when the power is removed. Of course this is all happening so fast that the loco goes the same speed on the track, but if there is any play in the gears between the motor and the drive axles, then they will hammer and make noise. I believe that this is solved either by a big flywheel on the motor, or better yet by a capacitor in the decoder itself. The capacitor will even out the voltage going into the motor and thus the gears don't hammer against each other making noise. In a DC environment, there is always constant torque on the gears so they have no chance of making that hammering noise. As a test, replace the decoder in your loco with a high quality one. If it doesn't make noise any more, great. If it still makes noise, then apparently the more expensive locos have a more tightly meshed gear set. -Lance
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