Questioning a contractor

We have recently had wind damage to our shingled roof, insurance is paying to replace it. I talked to a contractor who suggested metal roofing. He said it could be laid directly over the existing shingles since there was only one layer. So my questions are, is that OK to do? What are the risks? What kind of sounds (from storms and such) would be more noticeable than with shingles? Is there anything else I need to know about metal roofing, since I really know nothing about them? Thanks!