Seagate hard disk making ticking sounds. Failing or defect?

Hi,
I have a 500GB Seagate hard disk which started making a weird sound when the PC boots up. There are no errors when the hard disk gets tested and no visible degradation on it's work process.
It's a ticking sound which appears soon as the PC gets booted and while it is running.
Does anybody know the reason for this and could it get worse?
It has been like this for at least a month now with no degradation effect to it's content or work capabilities.
Much appreciated,
G,
