J666bmx

hey guys im a mechanic my friends 350z begun a knocking noise yesterday after it became low on oil , but as i dont know much about the engine itself i was wondering if anyone can recomend a nissan specialist in the midlands area that is reputable as it sound like a hydraulic tappet but dont want to start taking the engine apart, would really appreciate any help you can give, also if anyone on here is clued up on the 350z and lives in the miltonkeynes area and take take a look , that would also be great