Nissan X-Trail knocking

I have a 2001 model X-Trail which ran 130000km perfectly fine until one day the radiator leaked and I noticed engine temperature going up. Replaced it and went fine. After a few thousand kms in hot conditions the engine used to shut off while running. Nissan replaced the ETC and the CKP and went fine again. Now it is 148000km and I noticed that at high speeds the car jitters and vibrates when I press the gas> Recently I took it back to Nissan and they replaced the spark plugs, topped up engine oil and said that there may be an oil leak in cylinder 3. To return for check up after 1000km. Today I took the car for a 150km X 2 trip. On the first half one way the car started knocking at 120kmph when I pressed the gas when I was almost reaching destination. Reached destination and after 3 hours I was returning back home. Same thing happened, when half way the return trip the car started knocking again. I feel the car starts knocking when the engine is hot. Temp gauge shows normal. What is happening here? Nissan said that they may have to open up the engine if cyl3 shows problems. Do you think it is the engine of one of the sensors acting funny again? Please help.