Terrible service

I left my nissan interstar with nissan germiston almost 2 months ago to check a stop light issue ,a month ago they say oil pressure is abit low and i need to change the oil pump, all this time after test driving it my engine is still fine. So now they replaced the oil pump , took it for the test drive , everything was still fine and the day i go to fetch it ,they say a knock is coming from the engine and they dont want to take responsiblity for this when i left a perfectly good running vehicle with them. My vehicle is still at nissan and no one can give me an answer to move forward on this matter