What about the noise when it rains?
The "metallic" noise doesn't transmit well through the roof decking at all. There's a tiny bit more noise, but you really have to listen for it to even notice.
Like I mentioned above, for standard overlap joint metal roofs, there's typically a ~1" air gap between the metal and the roof decking itself. My Amish guys used rough cut true 1"x4" boards for the gridwork. There HAS to be an air gap for this type of roofing. It's a huge benefit in the heat of summer as the majority of the sun's heat is either reflected by the metal roof, or pulled away from the roof decking below the metal in the air gap space, by convection and exhausted out the ridge vent.
The more costly standing seam metal roof attaches directly to the decking with an insulation barrier in between that is going to muffle the sound and prevent heat transmission.
Again.. Metal roofs do have some advantages over shingle. Let me just give you the bads: If you have lots of roof penetrations and valleys they may not be the best option for you. I don't have faith in the long term sealing integrity of the surface mount boots that are used for penetrations.
Snow load is another big problem that people who've never had a metal roof do not understand. The snow that builds-up is extremely destructive. Gravity takes it downhill constantly. It will bend and break off any roof penetrations (plumbing vents, HVAC vents, service entrances) if precautions are not taken. I've been there. It will tear off gutters and do soffit damage. You cannot walk on a metal roof to remove snow, so once it's up there, it's up there. All the snow rails on my roof are ugly but unfortunately necessary.
Overlap metal roofs also have all the screws that poke through the metal. Supposedly the rubber "gaskets" on them degrade over time and need to be replaced. I've never known anyone who had leaks due to this, but supposedly it's a thing and the screws need to be replaced at some point.