Check for any loose components including anything on the exhaust. I have seen this happen before where something was slightly loose and the right load and rpm would make it rattle.

Where is the noise coming from and what kind of rattling does it sound like?

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record