This is probably the #1 question we as metal roof installers get.  Perhaps you remember standing under a barn, shed or outbuiliding during a rainstorm. In these structures, the steel roof panels are secured directly to the rafters.  There is no sound barrier so it sounded loud when it rained. On a home, townhome or heated commercial facility, the steel panels are fastened to an underlayment, then decking, with insulation under that and a sheet rock ceiling which absorbs and muffles sound.

In addition, and this too helps muffle sound, some metal roofs can be installed over an asphalt roof.  Considering the layer of shingles, roof deck, attic air space, rafters, the insulation, and sheetrock ceiling, individuals who have metal roof say what they hear when it rains is the rain on their skylights and soft metals (gutters), not their metal roof.