It could be a million of things but on my car i got a great deal when i bought it because there was a knocking but when i was looking at it i noticed that a vac line on the supercharger was slightly broken and was starting to rot. Bought the car brought it home it was knocking in 2 or 3 of the cylinders went to autozone got a 2in line for free and fixed it. the car runs like brand new