MyBook Pro makes clicking noise

My MyBook pro 500gb external drive bought 3 yrs ago makes clicking noise. This happens only when the HD is kept stored for a some days. After a few days of non usage and connecting back again to my windows 7 pc it makes clicking noise and not detected. But after disconnecting the power for a couple of times. Its back to normal.

why is this happening?. is my HD becoming weaker?

Many thanks