Howdy,
Inspect the entire length of the exhaust for solidly mounted hangers. You are looking for two things; any direct metal to metal contact between the frame and the cab sheet metal, and any hanger welded solidly between the frame and exhaust. If the installer eliminated the rubber isolaters from the hangers the exhaust vibration will be transmitted directly into the frame. Now you have a tuned cavity (the cab) connected to long vibrating tubes. At resonant frequencies you will be inside a drum. Worn out cab isolators will exacerbate the problem. If the installer welded a strap to the cab, eliminate it entirely.