Ideas how to quiet my cabin drone noise please

[joedoh]

its important to note, the problem is not the metal reflective surfaces of the cab nor is it the lack of sound deadening. this is a problem of resonance, where the frequencies are amplified because of efficiency (thats actually the roundabout definition of resonance).

a drone noise is most apparent in the frequencies of 250-500hz, which have a wavelength (the actual length of the wave) of 30-50 inches. so if you have 30-50 inches of uninterrupted pipe with a wider flange at both ends, you will likely have drone, because the standing wave can just sit inside the pipe and resonate.

I have had drone on modern trucks, my colorado was lousy with it and almost made you physically sick. breaking that long stretch of pipe between the manifold and the muffler with a center resonator fixed it right up.