20 years ago I worked (2 years was all i could take) in an appliance
factory. one of the lines I did was dishwashers. These we used
antiresonation material on...tarry sheets about 4mm thick, heated to soften
in an oven. These had a side with gladwrap-covered adhesive. Peel wrap and
stick on, press down with a roller and voila...soundproofing.

I used to take home rejected parts, and managed to amass a considerable
number of these sheets, with which I planned to quieten my 57 panelvan.
Never did it, but I bet I still have them floating around somewhere. Not
that my Caravelle is noisy...in fact it's a quiet vehicle, but there is
still some road noise I wouldn't mind getting rid of.

This stuff is HEAVY, and i would not be at all keen to put it on the roof
panels (may cause them to sag and kink, also unbalance the handling).