Strange "TINK" noise

I have a MBP 15 2.4 4GB RAM.

Im getting a strange tinking noise which is random but seems to happen more when surfing. I know this sounds crazy, but to me its randomness sounds like the windows pop-up blocker frequency.

However, it does not only happen when surfing. have a look here: YouTube - Mac this is a video i took on my iphone which shows the noise happening three times while the laptop was sitting still on a tabletop.

Im not sure if it could be the drive heads parking but is this possible if the laptop is being used, as it happens when the laptop is in use too.

And i also cannot think that it could be the Sudden Impact Sensor as it happens when the laptop is sitting securely on a table.

Any advice would be great!

CK.
PS speakers up nice and loud cos its only a quiet noise.

Certainly does sound like it is the HDD heads. Your MBP will read the HDD from time to time even whilst you are not doing anything on it.
If you can get your hands on another HDD and clone your HDD onto it, you may be able to see if it is your HDD.
The only thing I can say for sure though is backup backup backup!

HTH
- Simon