My fan is just to loud to be a good "white noise" machine at night, and it also has the thermostat and when in cycles off and on it will wake you up. I run a fan in my bedroom all summer just for the noise effect but this vent fan is overkill. The fact that it is about three feet over your head probably makes it worse. I asked a good friend with an electronics background my original question and here is his reply:
"Your fan is 26w at 12v. Watt's law (you get to lean something new!) states I(current) * V(voltage) = P(power), so 26w / 12v = 2.16amps
Now, we know the current and the voltage, so we can calculate the resistance. Ohm's law is I * R = V. So we can take 12v / 2.16a = 5.5 ohms. The motor has 5.5ohms of resistance.
If you put another 5.5 ohm resistor in parallel, the current will be 1/2 of the 2.16 amps, and theoretically you will have 1/2 speed. The friction of the motor and some other factors will effect the actual speed, but it will give you a quieter motor. The power requirement of the resistor is calculated as 6v * 1.1amp = 6.6 watts - round up to 10watt resistor for safety. In case your wondering, the 6volts is because the motor and resistor are equal, so together they have 12v across them, with 6 volts across each. The 1.1 amps is because we doubled the resistance (motor + resistor) to 11 ohms. 12 volts / 11 ohms = 1.1 amps.
Personally, I think I would try installing two fans with a switch that would connect them in series, parallel or off. this was you don't need a power resistor, and you will still get similar airflow (to one fan) with less noise. If you need to, I can send you a schematic for that."
So there is a theory course for us all today. I spoke with him later and the thought came up that a good starter experiment would be to try two old automotive ballast resistors first! We used to race bikes together and those were the key to being able to use car coils in our bikes. They essentially drop the 12v from the battery down to about 6v. Dontcha love cross referencing hobbies like that!
But back on topic, I'll look at the Ohmite site too. Heat is always a concern with this stuff, so an engineer designed solution might be best. If you look at your fantastic fan, you'll notice there is a cut out for airflow around the speed switch. I bet that factory switch gets pretty warm and they thought it best to use some of the air movement to keep it cool. Don't want to melt anything!