Ideas on what's causing my click/tick/rattle noise?

I know that tracking down the source of a noise can sometimes be frustrating. Friend of mine who runs a shop for Studebakers had one with a strange noise that he spent hours trying to figure out. Finally decided it was something in the gas tank. Turns out it was a golf-ball sized lead ball that the owner had put in the tank because someone told him it would add lead to the gas. So it was just rolling around loose as the car accelerated, decelerated, went around corners, etc. The owner was hard of hearing, so he never heard the noise, but it drove my friend crazy until he figured it out.