If that's the fan, it could be it's turning too hot inside. Things to check:
- are the fans clean and do they work (all fans)
- what's the CPU usage (check in task manager). If it's consistently high when you're doing nothing special, something is wrong. What process is causing that high CPU usage?
- how's the heat paste on the CPU? Refresh if necessary.
I am running on Windows 8.1 OS and my computer sometimes restart suddently when I think something too loud and I don't know why?