Can anyone tell me if this is normal?

[Chamberpains]

A lot of the cm870 and cm871 air compressors knock\rattle while building air both new and old compressor's. The timing can be lined up perfect and all. Then stop once the governor kicks out. Its sounds like a definite engine knock. Its normal on an ISX. They just knock when building air. Are you sure your not chasing a perfectly normal air compressor noise?