You're probably looking for Intensity (-I) which is a value of -10 to 14 (or 0 to 20 for scrypt) that sets the aggression level for mining. You can also set the intensity to D which will set aggression dynamically to ensure you can still use the computer while mining (without lag). The lower the intensity, the lower the hashrate and temps (and therefore fan speed). (I would not suggest using -I D for scrypt mining, your hashrate will drop down to very very low numbers).
If you're using Windows, you can also use
CGWatcher to lower or raise intensity at certain times, intervals, when the computer is idle, etc.
