Missing two of the black "feet" at the bottom of my Macbook Pro/Started making a clicking noise inside
The feet help air to get under your Mac and cool it, which is essential because heat destroys just about everything a computer needs to keep working, like the processor or hard drive. The clicking noise is probably your internal harddrive beginning to fail. You should back up your data immediately and prepare to replace your hard drive, as clicking is often a good indicator that the drive is about to stop working. You can check the drive's SMART status using Disk Utility.app, but it may not report anything amiss until its too late.

You can find detailed instructions on checking the drive's SMART status here, but the meat of it is accomplished just by stealing their image, which I'll link to below.