The roof on our house is standing seam on top of 8" SIPS - Exterior OSB sandwiching 8" of rigid foam on top of the ceiling - 3/4" pine. We hear the rain JUST FINE. I would have thought that 8" of foam would be pretty sound deadening but nope. I think you could need to have some air gap between the metal roof and the insulation.
You've confirmed with your experience what I've been thinking all along. We had a tin-roofed shed on my folks' farm and it was really noisy when the rain poured. Can't imagine how deafening that would be to chickens in a small coop space.