Hi I have a 2005 Malibu Maxx 3.5L v6 and I rolled about 60 000 Km with it. I hear a not usual vibrating metal sound when I press just a little on the accelerator or even on idle when the engine is cold. Sounds a little more like knocking than rattle (not sure, I will try to film it). The sound disappears when I give more gaz. Please help. If you have any ideas, please post.

Sorry for my bad English.
Any help is really appreciate
Anthony