"The most common cause of a clicking hard drive is when the drive heads are hitting an internal head stop. This is often called the click of death. This could also be due to a faulty circuit board on the hard drive. If it's clicking it either does, or soon will, need to be replaced"
If possible, back up all your important data now and get a replacement hard drive. It will fail, it's just a question of when. It will also continue to cause errors and crashes.