How loud is the noise? Because a plastic bottle popping back into shape is pretty noisy, whereas most HDD noises tend to be more on the magnitude of someone flicking the prongs of a plastic comb.
It's pretty loud, but it sounds muffled/contained if you get what I mean... imagine that you have an empty computer tower and then drop something small on the bottom of the case, then you get a plastic "clonk" sound which sounds muffled.. it's really hard to describe. But it sounds nothing like the click videos I have found. Because those sounds click way more and do not have as much bass ^^
Actually I've had annoying clicking sounds coming out of one of my DVD drives before at computer start. Popping the Disk out and back in and everything returns to normal.
You have to check though, the Hard Drive isn't the only thing with moving parts.
My initial thought was that it was a fan.. but I'm 99% sure that I can hear the sound from where the hard drive is, and it does occurr when I use that very hard drive
Well, if you're worried about it, you should get all your important data off of it. With HDTune you should do both a health and error scan. On the error scan, if the entire screen isn't green, or seems to hang at any spot, there may be a problem (a click of death won't necessarily stop it from being all green, but it should slow it down). The Health scan is a SMART test, and is just a precautionary thing, a bad HDD can easily die with a clean bill of health from a SMART test
It's very new, so I've got almost nothing on it.