Since I let my G4 Powerbook's battery run down, I hear a loud CLICK SOUND at regular intervals, that I never heard before. It sounds like a back-up click. Is this a problem?

After letting my PowerBook G4's battery run down all the way, I hear a loud metalic CLICK sound every so often.  Not sure if this is a problem or not. Should I be worried, or have it checked out?

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