Talk to me about air compressors...please

[Chamberpains]

My cm870 knocked while running and would be silent while not. This was both on a 1.1 million mile engine and a fresh cummins cert. Reman. Both were definitely visually verified and done to exact cummins install spec. By me. I chalked it up to the nature of the beast on this one. I ran both engines for a total of 700,000 miles with no compressing problems.

My 600,000 mile cm871 is smooth and silent always. But I've heard plenty of guys say the same knocking in theirs is normal.