Chattering Hard Disk on 27" IMac

I have a 2010 27" IMac with a Hitachi 2GB SATA hard drive which is now making a chattering noise and the computer now seems much slower. I have tried doing a diagnostic test by pressind the D key at startup but it won't carry out the test. I have verified and repaired using Disk Utility but it makes no difference. I have tried to do a Time Machine backup but after an hour it had only managed just over 500mb. I assume the hard disk is failing and wondered is there any other way to back up the disk before it fails? Any other advice to overcome the problem would be greatly received.