Rattling 04 250 Hpdi

COUPLE OF MONTHS AGO I HIT SOMETHING IN THE WATER,AND DAMAGED MY LOWER UNIT.REPLACED
WITH A BRAND NEW ONE.FIRST TIME IN THE WATER WHEN I PUT IN GEAR IT HAD A RATTLE,ONLY IN GEAR AND AT IDLE DID IT RATTLE.HAD THE DEALER ORDER ME A NEW LOWER UNIT.AFTER INSTALLING THE NEW ONE
SAME PROBLEM.I CAN TAKE OFF THE PROP, AND THE RATTLE STOPS.
THE DEALER TOLD ME THIS NOISE IS NORMAL.BUT I HAD NEVER HEARD IT BEFORE?
COULD SOMETHING HAVE COME LOOSE WHEN I HIT
WHAT EVER IN THE WATER?ANY THING I CAN LOOK AT OR CHECK?
THIS RATTLE IS DRIVING ME CRAZIER!!

[boats.net]

Could be prop chatter, try a lightweight aluminum prop if no noise with alum prop then it's prop chatter which not much you can do about it.

Thank You For Your Help

ONE MORE QUESTION:I HAVE A LIGHT WEIGHT COMPOSITE FIBER PROP FOR EMERGENCY USE, WOULD THAT WORK TO SEE
IF IT IS PROP CHATTER?

[boats.net]

Yes that will work.