Problem with my Cummins ISX

Hello everyone, I'm new here on forum and I need your help.
I have a Cummins ISX that knock at 1450 -1500 rpm. At the last service, they have completely overhauled the engine, replaced one injector, replaced the camshaft on the valve train, clearance checked the timing gears (all within specs), new head. Still have the same exact noise as before and the men from service said they found no damage in pistons, pins, cannot figure it out.

Somebody told me to use a dish tool to cut out injectors,and if i need to isolate the cylinder that has the knock, it sounds like an injector causing the issue once I can isolate swap the suspected cut injector with another cylinder, all injectors should be changed at time of overhauls. Or I'll risk damage to the overhauls engine. I know that an injector only has a life span of 500,000 miles.

Do you think is too difficult? What you recomend me, to buy the service manual for my truck [TradeBit] and to save money by repairing it myself, or you recommend me to go on a proffesional service and to solve this problem.

Thank you for your attention!