That clicking noise your hearing is the read and write heads of the hard disk.
If when in use you get a Loud Tick,Tick Tick noise its the read and write heads of the drive.
But if the drive is old as said above its a sign for you to back up all of your data quick.
And buy a new drive.
The magnetic platters are on there way out.
My seagate 7200.8 400GB HDD has always made noises when it runs. I thought it was normal. Its been running for 2 years with no problems. Always a good idea to make regular backups if it has anything important on it.