Mine did that when new also. I put a strip of "stick-on" foam insulation along the bottom inside edge of the flap. It's about 1/4" wide and 1/8" thick. Been on there for 8 years now - no rattles and it ventilates the stove and micro just fine. I suspect it still flaps some, but it doesn't make any noise...
'04 Journey 36G - Cat C7
'04 Honda CR-V