CM871 rocking back and forth at idle

[grainhauler]

New guy to the forum and fairly new to the industry. The truck is a 2008 model that my employer bought a couple months ago. The motor rocks back and forth at idle just enough to get on my nerves, the rpm holds steady and it sounds ok. As soon as I apply throttle the rocking stops. Other than the idling issue the motor seems to perform well. The motor was overhauled about 70,000mi ago by the previous owner and was demandated at the same time according to the service records of the owners local shop. Any ideas as to what is causing the issue and if it is related to the demandating? Thanks in advance for any information or thoughts