Just a single click, like it's turning off or turning on. If I use my PC for a couple hours I'll hear it once or twice. I have 3 HDs, I can't tell which one it is but 2 of them were in my last Windows XP system and neither made the noise. So it's probably either my new 1TB HD or something to do with Windows 7. It doesn't seem to always coincide with me accessing the files, but it does sometimes.

Is this some power save feature? Or is my new HD about to take a dump?

Sounds like power saving. If you dig around in the Control Panel you can find it an shut it off. It's supposed to be better for hard drive life anyway.

I had this problem, eventually the clicks became more frequent and then the hard drive became unreliable (computer crashes) so I had to buy a new one.

I've encountered it a few times before, my friends and I call it "the click of death"
Find out which one it is, back-up everything that's on there, get a new one.

It's impossible to diagnose on a god damned forum.

All hard drives make noise. Some make a click with the heads park, and it's perfectly fine. Some of them make noises when turning on or off.

If you turn off power saving and the noise goes away, don't worry about it.

Furthermore, a hard drive can fail without ever making a noise, so if you aren't backing up your data all the time, you don't care about the data.

JM, in most of those scenarios they would've been making clicking noises before, instead of only just starting to do it now.
(if it's the third you might have a point, but I've never heard a hard drive clicking without it being a bad sign)

My hard drives will do this, sometimes even when I've already been accessing their contents. I'll get a delay reading data, then 1-3 seconds later I get a click and it resumes.

Can be pretty annoying. Seems to happen with content on all 3 of my drives, as well.

It's possible that the drives are being turned off and the delay is how long they take to spin up. That sounds pretty bad, but it also seems unlikely that three drives would fail at once.

There is a change of OS involved too.

Yes, yes there is...

This happened to me once.

If it was a death click it'd be happening far more often. If it only happens occassionally (like when you access something after long periods of not) on a certain drive(s) then it's likely powersave. You'll usually hear them whir up right before it clicks too.

It could either be calibration or the heads crashing. If the heads are crashing you'd notice other problems, and it would be more frequent.

So do mine, Windows spins them down when I'm not using them and spins them back up.
Having 3 drives sucks, as soon as >3 TB drives are below 170$ im getting one and replacing all 3.