Hey, I have a 1994 Volvo 850 Station Wagon. Whenever I start it right up without having driven it for at least four hours or more, the engine knocks really really loud and the car sorta shudders, but it still drives. After driving awhile the knocking goes away. And if I drive the car again within that 4 hour span of just driving it, the engine does not knock at all. Any ideas?

Thanks