Clicking sound usually indicates that the hard drive is having some mechanical problem. It could be a physical problem with the components of the hard drive. First thing is to take a backup of the important data that is currently on the drive and complete the data recovery process as early as possible. More the strain the hard drive takes, the faster it dies.Not all clicking noise is related to physical issues, some could be even due to firmware related issues which can be fixed by tools provided by the hard drive manufacturer. However, it is recommended to take a backup as soon as possible.