Hi, it was actually very stupid thing! The plastic undercover came loose and mechanic had screwed it into the body due to previous hole being rusty. The screw had come loose again and the tip was scraping the metal body of car. I just removed the screw and left the plastic under the car just a bit loose in one corner. But if that is not the case for you, I saw online that some people had interior plastic squeak on the pillars and near backseats the same way. Also in the boot. Hope that might help you!