I replaced the old and worn fan in my 45S color head with this variable speed model, was easy to wire and even on high, I saw no ill effects from test strips done at 20x24 magnification when looking at them with a 15x loupe when dry or the neg in the grain focuser:

http://www.acinfinity.com/speed-cont...-fan-kit-92mm/

I did use some 3/4" high density grey foam as mounting block to help reduce vibrations further. I also used aluminum and black gaffer's tape to seal it off. Overall, a variable speed fan makes a lot of sense for this application, I feel it is perfect...