I used some Dynamat-like stuff I got from a marine supply place. They use it on fishing boats that run to Alaska and such. It's expensive but it does cut the noise and heat considerably. That said, my cab is still too noisy on the hiway. There's tire noise, exhaust noise, gear noise and biggest of all wind noise. With the camper off, the noise level drops to tolerable-without-ear-plugs. Same is true pulling a trailer -- most of the wind noise is behind me even though the engine is working just as hard.
