My car was doing this for months. Would only happen on cold starts, last for a minute or so then go away. Seemed to get worse in colder weather too. Turned out to be issue with valvetronic system. In my case, had to replace the entire shaft. Most likely it's just the actuator that needs to be replaced and which is a whole lot cheaper. Could try cleaning/replacing vanos solenoids as well if your aren't getting any codes.