2001 Chrysler 300 Rattle

There's no way to diagnose an engine rattle without hearing it and standing next to it, but if the battery was replaced, the Engine Computer is going to have to relearn "minimum throttle" before it will know when it has to be in control of idle speed. Until that happens, you won't get the nice idle flare-up to 1500 rpm at start-up, the engine may not even start unless you hold the accelerator pedal down 1/4", and it will tend to stall at stop signs.

To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.