Mattie, can you come back to me re this funny noise.

[Big Jenny]

I didn't understand what you said about the heat sink on the CPU, or something.

[mattie]

If you don't fancy delving into the guts of your PC, ask your friends to have a look inside, to check that all the fans are working.

On top of the CPU is a large metal grille with a fan fixed on top. It conducts the heat away from the processor, and the fan cools it down.

Any build up of dust in this grille can impair CPU cooling.

[dodgyshirt]

A build-up of dust would not cause the noise you describe, it is more likely to be a bearing overheating on one of the fans, either the CPU(processor) cooler or case fans.The frequency of this occurring would depend upon the length of time your PC is switched each time.

These are fairly easy and cheap to replace.

[qapmoc]

Hi Jenny, the big question is are you prepeared to open the CPU casing and check out something inside -
NO = take the CPU to a repair shop
YES = shut down the PC, take out the mains power connector, press the power on button (this will discharge any power stored in the power supply), open the casing and look inside, there should be a small fan fitted to the top of a slotted piece of metal which looks similiar to this , Now inspect the slots below the fan, if they are filled with dust you must remove it with a very small brush and a blower (hair dryer set to cold/cool?). With the casing still open put back the main power connection and start the PC, look at the fan and make sure that it is turning. if it is not working then obtain and fit a new fan.