Fan too fast.
 

I've mounted a 40mm 12V fan to my 060 heatsink to make sure it stays well within acceptable operating temperatures.

The problem is that it's a bit noisy. It spins very fast and somehow just manage to hit the right frequency to resonate with the heatsink. So, I'd like to lower it's speed a bit but don't know what's the best way to do it.

Add a resistor to the 12V line? What kind of resistor do I need? Or is there some other way I can do it?

Don't need to lower the speed of it very much, maybe 10-20% is probably more than enough.

When I had a noisy fan in my SCSI box I changed the input from 12V to 5V - it was ideal in my case. But a resistor cold be OK - just count from the fan specs.

You could use a manual fan speed controller, I used to use one on my BPPC setup, only cranking it up under load. Note that this was in addition to the original PPC fan.

Never thought about changing the input to 5V. I'll see if that's feasible to do and if the fan actually works on only 5V.

The other common trick is to run it at 7V - i.e. 12V connected to +ve and 5V connected to -ve

I have no idea how to connect that up. :)

You have for example three leads where black is ground, others 12 and 5 volts . 7 volts is between the two other leads.

In other words, connect the red wire to 12v (already is) and the black to 5v. Then you get 7 volts to the fan.

Ok, I'll see what I do then.

Fan speed controller or even using a potentiometer could work to adjust the fan speed? I put the latter of these on a child's toy gun to stop the volume being so damn loud!

If it's a low quality fan it can make sense to buy a better one. They have different rpm at the same voltage, so maybe you find one with lower speed and also some have better bearing that others. It's harder to find reliable specs for these small 40mm fans than for bigger ones, but at prices <10€ it's worth a try.

Putting a potentiometer in series definitely works. Try small values like 470 ohm to start with.

OP: Get an 40mm Noctua fan, it'll be silent at max speed.

Got this fan from Amigakit. Haven't been able to find any 40-45 mm fans in Sweden, not even Noctua ones.

But, I've been thinking about removing the heatsink and just attaching it with some doublesided tape in the corners. Should cool a 060/66 enough and the tape should dampen any vibrations.