40mm 24v. The stock one is very noisy. I am actually using a 12v fan on the 24v output and it is quieter than the stock fan even though it is probably running 2 times its rated speed. Some very knowledgeable electronics guys say that that fan isn't even needed because the mainboard doesn't get that hot. The fan actually just blows some air over the stepper drivers mainly.


http://www.digikey.com/product-detai...208-ND/2621113

This one will work if you want to try it. Not sure how loud it is though compared to the stock fan.