I have had the same problems with my Dell XPS17. The fan would go from being a quiet background noise to an annoying whir every minute or two. This seems to be something to do with the NVIDIA graphics processor being used unnecessarily.
To see the NVIDIA GPU usage:
1. Go to the Desktop. Right click and choose NVIDIA Control Panel from the menu.
2. Choose the menu Desktop and enable Display GPU Activity Icon in Notification Area.
3. Click on the GPU Activity Icon (bottom right of screen) and see the programs that are using the GPU.
In my case, Syncup, toaster.exe and a Weather widget were all using the GPU. This all seemed unnecessary and should be fine on the Intel integrated graphics.
So in order to only use the GPU when necessary, I have changed some settings. It seems to have helped.
1. Go to the Desktop. Right click and choose NVIDIA Control Panel from the menu.
2. Choose 3D Settings -> Manage 3D Settings.
3. In the right panel, choose the Global Settings tab.
4. Change the Preferred Graphics Processor to Intel Graphics and click Apply in the bottom right corner.
5. In the right panel, choose the Program Settings tab.
6. Select programs that need high performance graphics such as games and set the preferred graphics processor to NVIDIA, clicking Apply every time.
7. Restart the computer just to make sure.