Knocking noise

My vehicle is Mercedes e270 5 cylinder diesel engine I got injectors repaired last week by a local mechanic. Earlier no noise is there expect missing problem but after he starts the engine the missing/miss firing problem is gone but a knocking sound (same as a tractor do) starts coming now he is unable to diagnose the fault but he can repair it if he got exactly from where this sound is coming maybe you can help
(I can send a send a video if required)