You can use the search function “Fan Speed Controller” in your FCOM and you will find it. It’s actually at the very beginning of the FCOM in the General Information. It does not make a big difference but still noticeable. Only few aircraft in our fleet have it; Unfortunately not the latest models anymore; They did not buy this option..From what I understand without this option, the fans will just run at high speed continuously regardless of the temperature. But with this option and when the temperature is under a certain threshold the fans go to a slower speed and that’s when you can notice the difference.