Yes the clicking sound your hearing is indeed the internal hard drive failing.
Call up customer services to advise them and they should replace the box for you, if they do not then as you have been with them for a few years threaten to cancel if they do not replace the box, stick to your guns and get right through to retention's if the initial customer service adviser is of no help.
Also if you get as far as retention's to get a new box also mention the cost of your package as you will probably get a discount on your package too!
I'm very sorry to learn about the problems you're experiencing with your TiVo service recently, I've been able to locate your account to run some tests on the set top box from our end for you.
We're detecting some errors and I'd like to arrange for a member of the team to come and take a closer look, if you're available? I've sent you information regarding this in a private message, which you can view by clicking the envelope on the top right of this page.