Hi Anthea,

If it's making a strange noise then it should be looked at right away by a good mechanic. 

Other than that, cambelts have specific replacement schedules. That varies by engine, make, model and year so we'd need to know all those things before being able to look it up for you.

Likewise, the cost varies depending on the car and who is doing the work. I suggest you ring around a few places to get a price if you don't already have a regular mechanic.