Originally Posted by Kevin.Cook
Could be a few things, such as an exhaust leak. Sometimes these only present themself under a certain amount of load. Have you done anything to the exhaust?
You can further confirm this if you turn the car on, and plug up the tail pipes. Then listen under the hood and around the car.
I wouldn't suggest plugging up the exhaust, because if there is no exhaust leak, that can be bad news for the engine, it's easier to just crawl under and look at the couplings.
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