Cold Start on new 318

Hi,

I recently purchased a brand new 318i (just today it's one month old). Recently when I start the car after not using it for some time, I hear a sort of like chain rattling noise when the engine first accelerates. It only lasts 1 second and doesn't make a sound anymore afterwards (even after 1-2 hours if I start it again).

Any ideas what it could be? I have noticed that it does it when the car is cold.