I have the same 3.0 BiTDI and I too hear the exact same rattling noise. My mechanic said it is normal too but it is really been bothering me. I have tried other V6 cars like Mercedes Benz ML 320 and even though it is a much older car and engine, it sounded smooth. I even bought a new Oil Pressure Control Valve: https://www.csabastefan.com/en/audi-v6-bitdi-cvua-oil-pressure-valve-replacement.php but, having a hard time replacing it because of the very tight space between it and the radiator fans.