theres an awful noise coming from the computer it comes and goes any idea what it could be? whirring type noise?

Hard drive possibly?
see this
If it is, make sure you back up your drive
When you do hear the noise, you should try getting a little closer to your computer and listen near the Hard drive.
Also possible- the fans. Try getting close to those.

so they need replacing do they i guess. Is that a big job.

This is what I would do.
If you have a neighborhood computer geek you trust, I would get him to listen to the sound. Or if you could describe it here in more detail that might be good too. Anyways, if you think there's a chance that it could be a Hard Drive failure, I would go ahead and put in another hard drive. I don't know how comfortable you would be doing that. Your local neighborhood geek would probably be willing to put one in for a lot cheaper than a shop. Hard drives are very very cheap. But first things first ---
Can you describe the sound in more detail

a kind of whirring/vibrating sound can start off soft then gets louder, comes and goes.

Hi Ladyjane
Firstly how confident are you at looking at the inside of your Computer try to give as many Details as you can. Your problem could be as simple as your Fan needing cleaned to check it look inside the case and see if the Fan is dusty. It could also be your hard drive as previously mentioned. Have you had any problems with your Hard drive or anything else and have you changed anything recently. If you do look inside the computer case Please Do not Touch anything directly with your hands.

its gone away now strange.

Super! If it does come back, feel free to post here.