i tried out a lower RPM fan with manual knob and tach line. I only had one to test so I kept one original s7 fan at the rear. Manual knob fan worked fine, but I noticed that rather than start at low/quiet RPM for original s7 fan (as it typically does for about 45 seconds before linux/cgiminer initializes), the stock s7 fan starts at max RPM immediately after power up. I'm guessing that machine recognizes one fan is not stock and so it defaults to max spin on the fan that is stock.