I don't agree with with the description of "metallic" noise. It's not metallic. People who describe it as such probably have never heard a real metallic engine noise before
It's just induction/valvetrain noise. You hear it in all NA engines to and extent. You hear it clear as day in VTEC honda motor just before the secondary cam kicks in. Personally, I don't even hear it unless I pay attention to it. However, I can hear a knock with the radio on, in the middle of a conversation. I guess I'm just more sensitive to things that unfriendly to engines.