A clicking noise in an external hard drive or an internal hard drive is likely caused by high frequency vibration in the mounting hardware and/or a pending hard drive failure.
I would run a test on the hard drive and be prepared to backup all your data on that hard drive right away and transfer the data to a new hard drive or back the data up in the cloud.
A hard drive should not be making clicking noises so something is wrong or something is about to go terribly wrong and all your data will be lost.