" EZ Cool insulation is ...... closed-cell polyethylene foam core " honestly that may not actually be a bad idea. I sprayed my attic with 2-3" of closed cell. it got very quiet. I would imagine that it would mass dampen sheetmetal very well. its rvalue is pretty high ~6.5 per inch so a surface coat would only be . the downside is that its a mess before it cures and could be a pita when its cured and in the way.
Really though spray in bed liner or the like for mass would make things quiet too...