The clicking is the sound of the read/write heads moving back and forth to access data on the drive. Some drives are louder than others, but they all do it to some degree.
The read/write heads are mounted at the end of the arm you see in the photo linked below, and the arm moves back and forth as the platters spin.
It might just be something vibrating in my case. I only hear it now when i put my ear in the top back. When i put my ear in the mid front I don't hear it. It just makes me worried since the drive I have is a 7200.11 'Cuda.
That isn't normal... LOL! It's normal if it's getting worn overtime, but that doesn't mean it's necessarily REALLY bad, depending on how loud it is and how often it ticks, that's how you measure how bad it is. The loud long pause ticks is the one you have to worry about. Ticking is not the hd working properly, I'll tell you that right now...
Like if you start to feel your computer feeling sluggish for no reason, but it seems fast and responsive, it's probably your hd going bad. And it's a subtle sluggishness.
It depends on what the clicks sound like, faint rapid soft clicks that sound more like a clattering noise, are normal. Do you have a drive acitivity light? The light should be flashing in time with the clicking. Certain brands of drives are louder that others.
Loud sudden single clicks are bad news. They will sound more like someone tapped the top of the drive with a screwdriver. Usually these are accompanied by a momentary lapse or freezing of your PC. These can be just one occasional click, or a steady click.........click..........click..........click for a several seconds.
When you hear this, and you don't usually need to listen closely for these, as they are quite audible, it is a sign you need to save your data immediately somewhere else, and prepare to dig into your wallet.