Re: Ideas how to quiet my cabin drone noise please
 

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.

Re: Ideas how to quiet my cabin drone noise please
 

Agree. I moved mine back as far as possible. Also, installed sound matting in the cab. Reduced the drone considerably when cruising on the highway.

Re: Ideas how to quiet my cabin drone noise please
 

No drone with this setup. Idle and low speed sounds great, might be too quiet at speed.

Re: Ideas how to quiet my cabin drone noise please
 

Does a H pipe or X pipe have an effect on the sound waves?

Re: Ideas how to quiet my cabin drone noise please
 

sure it will, you are breaking the long standing wave. but sound is a funny thing, sometimes two waves will cancel and sometimes they will compliment and make it louder! that is why I assume long pipes in front of AND behind the muffler may not have a drone, the two long standing waves offset.

I did car stereo for a long time as a kid and it was always fun figuring out what worked for gain and what did not. I put two subs in the sidewalls of a toyota once (one on each side, pointed at each other) and they cancelled until you opened the trunk, then they complimented.

Re: Ideas how to quiet my cabin drone noise please
 

My mufflers are back just behind the cab, with dumps on the outlet it was horrible at low rpms. At idle the bed sides vibrated but droning went away at about 2k rpms. It does not do it now with full exhaust. There are a lot of bends to route the tail pipes over the diff and around the fuel tank. I assume the turns influence the sound waves as well.

Re: Ideas how to quiet my cabin drone noise please
 

I bet you are correct on the bends, in fact I bet it is different between mandrel bends and kink bends, the mandrel bend keeps the sound wave intact and the kink bends modify it.

Re: Ideas how to quiet my cabin drone noise please
 

Compression bends are said to effect the exhaust gas flow so they must effect the sound waves too, with my poor hearing I'd be hard pressed to detect a noticeable difference.