Loud Clicking Noise in REAR of my 2001 Ford Explorer?

Question

It makes a "click, click, click" noise in the rear of my Ford Explorer. It only happens when the truck is turned on, and it's annoying! It is above the back drivers side tire, inside the wall. Right in that area is also a button for the power locks, but I have rarely used that button. The noise stops when the truck is turned off.

Answer 1

Is that where the gas tank is?  It may have something to do with the with where the fuel line connects to it, if so.

Answer 2

If you have rear a/c its the blend door motor