The first word that comes to mind when you describe what you are hearing is OUCH! That hard drive has failed on you. The buzz and clicks are caused by a logic problem within the drive. The hard drive is trying to find a certain location on the drive and is operating the read-write head arm assembly starting at the outside and continuing to move it inward (buzzing) until it reaches the end (clicking) and then it resets (more buzzing) and starts the process over again. Any data that you have on that drive is gone unless you want to send the drive into a data recovery lab such as Ontrack (found at Kroll Ontrack recognized leader in legal technologies and data recovery...
). Be aware that such services typically cost around $1000. In any case, it's time to buy a new external hard drive. In my professional experience, I have found that Seagate hard drives are the most reliable, so I would definitely suggest you look at one of theirs.
Having said that, be aware that any hard drive will be prone to failure if you take it with you from place to place regularly, so I would recommend that if you intend to do that, replace the drive every year or so just to play it safe and not run as much risk of losing important data.
I hope this helps. Let me know if I can provide any further assistance.