Laptops tend to produce a lot of heat during its lifetime and the thermal paste put onto the CPU from factory begins to dry out. This means the heat transfer is reduced causing the revving of fan speeds.
1. Dissemble the laptop and location the CPU and Heat Sink. (Be careful to remove the power for the fan. NEVER PULL BY THE WIRE!!)
2. Unscrew the Heat Sink from the CPU and wipe off the old Thermal Paste. (Use rubbing alcohol and nothing else)
3. Reapply new Thermal Paste, no bigger then a pea size dot. (Don't worry it will spread as you reapply the heat sink)
4. Replace Heat Sink and reassemble laptop.