You can insert a rectifier diode in the fan's power supply line. That'll cut the voltage by about 1v without you having to compute a resistor value. Daisy-chain a couple of diodes to get a bigger voltage cut.

Since the rectifiers have a polarity, you need to insert them in the proper direction; if you've got it backwards you'll get nothing to the fan. Flip it around and it should be OK.

Chances are that if you achieve a quiet fan speed it'll keep things cool enough.