THERMOSTATIC AIR CLEANER....
Sounds like your car is pre-warming, their will be a flexible metallic tube going from the exhaust manifold heat shield to the air intake or air cleaner, where this hose attaches to the intake (underside) their will be a small round metal valve on top of the intake (coppery color) with a small black vacuum hose coming from it. when the car is cold and you turn it on the vacuum will activate this valve to open allowing warm air to be drawn via the flexible metallic tube from the exhaust heat shield (pre-warming) when the car is up to temperature the valve will close allowing fresh cool air into the intake. The round metal valve on top is not the problem, it will most likely be the butterfly which changes air direction inside the air cleaner it is metal on metal hence the rattle, you can remove the plastic piping that connects to the intake or air cleaner turn the car on when it is cold and shine a torch inside to see if this valve is operating correctly and or is the source of rattling. i have the same sound. i do not see it as a problem as it will last 15 minutes max. try and avoid removing any vacuum related parts from your car as it will be detrimental to the performance.
i hope this helped you and maybe someone else.