Vibration

If all you need to do to hear it is have the engine running, you should be able to crawl underneath to inspect the exhaust system shields which are a common source of light, tinny rattles. If you still can't isolate it, consider checking at the auto parts stores that borrow tools for a "Chassis Ear". That is a set of six microphones that you clip to various suspect points, a switch box, and a set of head phones. By moving the microphones around and listening while driving, you can zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool but most new car dealers have them. You can buy them too from the guys who visit the shops each week with their tool trucks.