Kai,

I tried something similar on an instrument I was building. What I learned is that many fans for PCs and instruments are not turned by DC motors at all, but rather DC brushless motors. A minimum voltage is necessary for the motor controller to operate. When the motor is started, the back emf is low causing a large current to flow through the windings. Voltage at the fan leads is now much lower than necessary for starting due to the dropping resistor place in series. Consequently, the fan will not turn continuously, but only pulse as the motor controller chip goes through it's protection cycle.

Bruce