I assume you're experiencing this noise on deceleration, sort of a "BRRRRRRRAPPPPPP" as you're coasting down to a stop? If so, it might be ignition timing that's a little late, or a slightly lean mixture as you're coming to a stop. If the noise is happening at cruise speed, or on acceleration, pipe length or the lack of a crossover pipe ahead of the mufflers might be the problem. Try advancing the timing a little, and/or add a dashpot to the carburetor linkage to ease the throttle closed instead of chopping it off abruptly as you decelerate.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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