What does a clicking sound mean?

One of your computers has hung in the middle of saving a large spreadsheet. When rebooting the PC, the BIOS screen hangs and a clicking noise is heard from inside of the computer. What is the most likely cause of the problem?

A) The power supply is not providing enough power

B) The video card is faulty

C) The hard drive is faulty

D) The memory has become disconnected from the motherboard

E) The elves that live inside of the computer are on break