One last idea just came to me... I may be reaching with this one, but hey, you never know.
If you are hearing this ONLY after it has been driven and NOT when it's cold, it could the exhaust striking against part of the undercarriage. What I'm getting at is this: the metal of the exhaust tubing tends to expand a little after it heats up from all the hot exhaust fumes. If the pipe(s) are only a millieter away from touching another part, they may strike the part when the vehicle is shaken. This would be easier to accomplish when the exhaust tubing is hot because it may have shifted closer to the strike point. When it's cold, and the metal is retracted, it is too far away to strike.
Just one other thought.