I came across this problem yesterday for the first time, and might be able to shed some light on it...
I work at a large UK university, and repair computers for a living. A student came in yesterday with a Macbook Pro presenting this exact issue - a single glassey 'tink' or metallic click sound was happening intermittently. It seemed to be coming from around the trackpad area, she reported that it might happen more when she was using the trackpad, although she'd heard it happen while typing, or sometimes even just moving the laptop.
My first thought was that it was a noisy hard disk, but after taxing the disk with a lot of applications and downloads open at the same time, it was clear that this wasn't the problem. It crossed my mind that it could be the heads 'parking', but the sound wasn't quite right.
I then thought it might be some kind of arcing, but this seemd wrong - the sound was happening when the machine was being physically touched, and didn't happen when it wasn't. The student went on to demonstrate how the sound could sometimes be made to happen by running fingers over the back of the laptop, under where the track pad is.
I removed the back of the Macbook and scratched my head for a while, then the penny dropped: the battery is under the trackpad, and this is the only thing that could be making the sound.
As it happens, the battery is wrapped in a stiff, plastic sticker, which isn't completely flush with its surface. Applying light pressure to the battery in certain areas replicated the ticking sound exactly - it was air bubbles trapped between the battery and its sticker.
There seemed to be no fix for this, but the student left reassured that the machine wasn't faulty.
I'm not dismissing any other claims that this sound might be caused by anything else, but in this case, that was most definitely it. Might be worth checking out if you come across it.