So I found this after some research:
"The Pentastar has hydraulic valve lifters and sucks at keeping oil in the top of the motor after it's shut off, so the oil typically drains out of these hydraulic lifters
. Unlike solid lifters, these need to be pressurized with oil to "self-adjust." The motor might need a second or two to get oil back into them after startup, and thus you hear a rattling for a few seconds within the lifter until it pressurizes with oil. It doesn't help that half the motor is plastic, which doesn't do anything to muffle the sound. "