My hard drive has just started to..

What operating system? How old is the hd?
If XP, you could run chkdsk.
If its held in properly with the screws and it sounds worse than it normally was, it could be on the way out.
Most of the manufacturers have their own diagnostics so that could be a good place to start.
If it's newish and if it is faulty, you might be able to get it replaced .

Just to be safe, I would backup important data. Download the diagnostic program from the hard drive manufacturers web site and run it. It should tell you if there are any problems.