Well, I am backing my data up and I hear a slight two tone click every now and then. Is that normal? I am using a 1TB backup plus. I can't remember ever hearing that before but I rarely use this drive.
This may just be the drive head on the disk coming to rest, in which case it won't be an issue.
However, especially if it is a new noise and it is persistent, it could be what is referred to as the "click of death".
If it is that, it is caused by the drive heads having to keep recalibrating and/or recovering from a read/write error on the disk, which can be sign of imminent harddrive failure.
Check here (includes an audio recording you can compare with your own drive noise): http://en.wikipedia.org/wiki/Click_of_death
and google "seagate click of death" for lots of information on the issue.
You probably want to look into backing up the data on the drive to be on the safe side, especially if this is a new noise that was not previously heard.