yokogsr,
Hmm...dong, dong, dong...could you characterize that a little more? Is it like a bell? Something a little more tinny sounding? Does the noise occur when you are driving or if you sit and rev up the idle to 3500? Have you revved the engine while looking in the engine bay to pinpoint the noise?
My first thought would be to check your heat shields on your exhaust components. But please give some more information.
Bogatyr