mkae sure your cabling within your system is clean and tidy, also make sure your heatsink didnt come lose or make sure it's properly seated. also using a can of compressed air try to blow out as much dust,hair,etc that might be within the case.
Make sure while checking these and performing this, that the compuer is unplugged from the wall.
You may even want to goto a local computer store and buy a thermal pad or arctic silver 5 thermal paste. amd some isoproplyl alcohol or articlean. and use it in a well ventilated area and clean off the top of the cpu and heatsink.
Then reapply the thermal pad.
And for paste, but a drop of if on top the cpu (Literally a drop) and spread it evenly with an old credit card or something, bu whatever u do DO NOT SPREAD IT WITH YOUR BARE HANDS!. and then reseat the heatsink properly.
If that doesn't work replace the heatsink fan and use a new thermal pad/ paste.