Here is my best guess based on the info out there.... Exhaust leak. My guess is that some where along the egr tube(not sure how they are routed on the 99+) if you can feal it in the throttle body while you are revving it. A minor exhaust lak may not make any noise at idle but can get really loud as more exhaust gasses are sent through it ie: revving it up.
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A good plan, violently executed now, is better than a perfect plan next week.
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