Sometimes, externals could make clicking sounds when there is not enough power supplied to them. I guess it will not hurt to use a different USB cable and attach the drive to another USB port.
In case the issue persists, I would agree with Phillip Corcoran and say that the HDD has failed.
I doubt that you will be able to access it using some software tools.
If the data stored on it is really important, you will need to contact a data recovery company.