Is it Knocking all the time? How many miles? What type of gasoline are you using? Do you do in between oil changes? Does it just knock upon start up? Lets start with these questions, and narrow it down. It might be your camshaft going bad. But do a search about this. There was an bulletin about these engines for the 540i years ago. It stated about customers complaining about there engines knocking. BMW told them that it is normal and will not cause any internal damage, and that it can be driven for many miles more. Don't know the details but, search it out. Good luck.