With the system off, inspect the CPU fan.
Usually it’s pretty obvious when the fan becomes lose. One of the plastic grommets will have popped off and the fan will look crooked and you will be able to wiggle it around with minimal effort.
If so it needs to be remounted. Remove the CPU fan and inspect the mounting grommets. Are they intact or are they broken and worn? Of broken or worn to the point they wont secure the fan, replace the fan.
You should also replace the thermal paste when you replace the fan like this:http://www.ehow.com/how_4777462_replace-cpu-fan.html