I'd like to chime in just in case you haven't spent your money yet. I have had a metal roof on my home for 11+ years now and it should be a lifetime roof, but I doubt it. There are two types of metal roofs, standing seam (most commercial installations are this type) and corrugated metal with thousands(?) of exposed fasteners. With standing seam there are no exposed penetrations. With what I have, the screws are fastened securely with some type of long life washer. One day about a year ago, my wife told me I needed to repair the roof, as she could see a row of screws sticking up above the metal. Sure enough, about a half dozen screws were about 3/4" out from the roof surface. I retightened them and hope they hold this time.
As for noise levels, I am thankful I have a remote for my TV set. Our house had open cell foam sprayed in the walls and on the top of the ceiling sheetrock and I hear the rain. It isn't REALLY loud, but it's noticeable. I would never go back to shingles, but I will suck it up and do the standing seam roof if I replace the one I have.
A side benefit of standing seam roof systems is there are mounts for PV arrays that clamp onto the upright seams and that means no roof penetrations for mounts. Maybe not an issue today for you, but later?
You may also want to consider this from the "above roof ventilation" thread:
http://ecorenovator.org/forum/renova...v-east-tx.html
It's obviously your decision and your money, so weight the benefits carefully before you choose, as some money is best spent instead of "saved."