The fans making a different sound before the system turns off indicate a possible drop in voltage. Could be a power supply that is dieing, or a crappy outlet that has voltage fluctuation. Plug a lamp into the other outlet next to the one that your PC is plugged into. When the pc turns off or the fans start making weird noises see if the light dims. Dont entirely rule out the heat issue though. You want to constantly monitor your CPU temp (or others too like vid cards or HDs). I use a program called Motherboard monitor. It displays the temp of your CPU in the task bar. You can get it here http://mbm.livewiredev.com/download.html
Threaded Mode