Hi bazza8952,
Have you made any hardware changes lately like adding memory, changing hard drive etc etc,or worked inside of your PC ?.
Firstly check your internal cards and memory sticks, power connections to all drives etc are fitted securely, remove and re-seat all,
Secondly if still the same, if you are comfortable, you need to switchoff PC and disconnect all hardware like hard drive, DVD drive etc etc including the power cables, remove any internal and external devices like graphics\Sound cards, printers etc etc. Then switch PC on, depending on what happens regarding siren sound will determine your next step\s, If no siren sound then connect a card or hard drive etc one at a time with appropriate power cable, checking again after each one.
Good Luck,
jodav :)