You know we can't hear it over a computer. There could be something loose in the exhaust system, from a rusted clamp to a broken substrate in the catalytic converter. A heat shield could be loose. Anti-sway bar links often rattle too. If you can't make the noise occur standing still, you'll need to have a mechanic put it on a hoist and search with a helper running the engine.
There is a tool called the "Chassis Ear" that can be useful too if you can find one. It consists of six microphones, a switch box, and headphones. You place the microphones on suspect parts, then listen while driving and switching between them. Be aware that many mechanics have never heard of this tool. You might find it at an auto parts store that borrows or rents tools.
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Sunday, August 14th, 2011 AT 1:08 AM