This kind of noise is nearly always due to the processor fan, but to be certain take the cover off the computer. Get a sheet of A4 paper and roll it into a long tube about 2cm diameter. Put one end against your ear and use the other end to locate the source of the noise. New processor heatsinks aren't expensive, but you need to get one to match your processor. If it is the processor fan that is making the noise, you can buy just the fan, but it is probably easier to buy a complete heatsink assembly - you can get them for about £5 - and swap the fan from the new heatsink to the one on your computer.
If the fans are get-at-able, you should be able to briefly touch the hub of each one in turn (take care not to touch other components) to momentarily stop each. When the noise stops, you've fould the culprit.