It can be the timing chain. They can go bad, but something that can cause it to chatter is low oil pressure. You can add a little more oil and see if it goes away. The tensioner is not hard to replace. I am pretty sure I did mine with clam on....from the bottom of the car.
EDIT: Let me clarify. The timing chain is the loud obvious rattle that occurs when the engine first starts up. If its just a light ticking, that would be something else.