This is exactly what my engine does except it's around 2,700 rpm according to the factory tach. Loudest at cruise. Goes away during acceleration and deceleration. I compare it to someone rapidly shaking a jar of marbles. I heard on Car Talk one time that the exhaust heat riser flapper can make...
Thank you very much for the info!
I have a 73 Mach1 with the 351 CJ, manual transmission and no air. Calibration number 15A. As you stated, that calibration number is not in Vol 6 of the Shop Manual. I went to the dealership back in the early 80s to see if they had any additional info, but no...