Alienware M17XR4 Fan Running High

I have noticed lately when I am playing games, the fan is working harder than normal. Sometimes it even runs hard when I am watching a video. 

I am wondering what I can do to fix this?

I am guessing that there might be dust build-up (I have had this laptop since the end of 2012). 

Any and all help would be appreciated.

Thanks,

Justin.