Just like the title says, this afternoon one of my WD hard drives started knocking, about once a second. It froze my system, the XP clock was about 3 mins behind when I noticed the knocking.
I reset my computer because it was knocking and it was still knocking while at the POST screen, and it took much longer than usual to get past the detecting hard drive phase. In fact, after about a min or so, I powered off and unplugged my data cable and power cable for the hard drive and I was able to boot normally and everything else was normal, except I have a missing drive now.
I am trying to google for a solution but I am getting a thousand different answers. Most relate to the clicking of death, but I've heard that sound, this isn't it, from what I can tell. That's why I'm calling it more of a knocking sound. It's like a thud, maybe once a second or two.
It may not be the usual click, but it still sounds like a dying drive. Maybe it's just one bad sector, but this is a point to take no chances. I wouldn't turn it back on until you have your backup/rescue option in hand.
I've used hard drive enclosures to get date off the drive in these situations, but your link on amazon looks like a clevery idea - you won't need to figure out how wide your drive is and can use it for different ones.
Just for completeness, I'll ask the bleedin' obvious question: While you were in there, you did verify that everything was rock-solid inside the enclosure. None of the support structures had worked themselves loose. There isn't another bay where the drive itself feels more solid?
(BTW, I completely agree with Cubs that, unless you've seen a likely cause, don't use the drive til you have a backup/rescue option! :-) )