System fan is running more frequently or at higher speeds than it used to when system was new. There are several possible causes for these symptoms - most common causes are:
Download and install the latest version of power manager from the Lenovo support site. Clean the fan and heatsink with canned compressed air. When cleaning the fan, be sure the system is powered off and direct the canned air into the vents at the side and or rear of the system.
If the system is outside of warranty and you are technically confident, removal of the system keyboard can often allow access to the fan and will allow you to clean the fan blades more fully.