Hi, I'm Greg (Indy-Tech).Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP.Rgds Greg.
Is the knocking noise also intermittent .
Is the check engine light ON or flashing when the engine is running .
Hi Terry ,
This sounds like a typical injector issue .see if your repair shop can do a manual cylinder balance test
to see if the injectors are the problem.
This is where the engine running at idle and hot is to loosen up one injector at a time until diesel comes out
the cylinder then will drop rpm and when you re-tighten it back up the rpm should stabilize do this to each cylinder ajy cylinder no even with the others is the problem injector .the knock will also subside when the bad injector is found.
Try this then get back to me ..