I've been playing with my 555 pulse timers and figured I'd post what I've done so far as there really doesn't seem to be a decent schematic online anywhere.
I didn't want any potentiometers, I don't want the owners turning the fan down. Just a simple 555 timer schematic for running the fan at a decent rate.
Runs off the fans 12V Brown wire so no need to wire from another source. To regulate the pulse peek voltage to 3V I used a common LED and it also helps to visually gauge the pulse frequency.
With 4.7K ohm at R1 it is still quite fast and about as loud as a 360 12V Mod. Combining resistors to make around 5K ohms or so should quiet it down a bit more. You can change the C1 from 1uF to 10uF but then you must change the R1 value accordingly.