Clicking is often a sound of a failing hard drive. If this just started I would be worried. Either way, now would be a good time to make a backup just in case!
clicking is the sign of a failing hard drive as the head is searching over and over again in the same sector for data that may be corrupted. This may be a head-park test when the system is powered up. If the problem does not occur during intense disk activity, I don't think you have anything to worry about. However, you may want to do a chkdsk on it and view the log file when you're done to check for any bad sectors that may have been found.