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I have a class C based on the M400 Dodge chassis with the 440 lean-burn engine. While accelerating, it gets really loud around 60 mph and quiets down at 65. If I accelerate hard it starts at a lower speed. Backing off on the petal reduces the noise. There is no chassis vibration or effect on the handling, just so much noise, I stay at 55 most of the time. Other than glass-pack exhausts, it is stock.
Have a Cross-over pipe added to the Exhaust system, it will quiet the loudness down considerable, and will even increases the power. Talk to your local muffler shop, they will tell you the same thing, if you haven't already. It's a quick and inexpensive cure, that really helps.
I had a 77 southwind with a 440 and it used to make a lot of valve train noise,i vacuum timed it and ran the highest octane gas i could buy and it did get quieter but still had some valve float type noise,always had oil pressure and ran great