Semi-frequent Marble Dropping sound from HD

I went away for the summer and when I returned yesterday, i noticed that while the Hard Drive is not in use, I hear the sound of a marble droppng every minute or so. I never had this problem before. And the machine is still running at great speed and has been for the past 48 hours with the problem. But I can hear the click/marble drop easily in a quiet room. At the very least, its annoying as ****. Its coming from the left speaker/hard drive area. Is this a common problem? Does it have anything to do with me not using the machine for weeks/months? Maybe it will go away after another little while of use? Should I be worried?