Won't tell you that Dyna-Mat's a sure cure, but seems like a good way to go. I've used rubberized roll roofing for essentially the same thing in my VW's - mold it with a heat gun - it's much cheaper than Dyna-Mat and you'll need a lot. The best it does is to dull the sound, but a big improvement in something like a Bus.

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89 300E
79 240D
72 Westy
63 Bug sunroof
85 Jeep CJ7
86 Chevy 6.2l diesel PU

"The object of life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane."
Marcus Aurelius