Dual pipes on an inline 6-cylinder have a 120-degree separation between exhaust pulses, and each pipe gets an alternating flow, and sound. At certain RPM, as you've figured out by now, the resonance in the pipe from end to end gets really obnoxious. You can change the resonant RPM, but not eliminate it entirely, by installing small mufflers (resonators) somewhere in the exhaust flow. The position of the resonator in relation to the main muffler will definitely affect the quiet and loud points.
Even with a quiet muffler, a dual-pipe six has a distinctive sound. The really obnoxious ones back when I was a teenager had steel-packs, not glasspacks. They were straight-through flow, with steel wool instead of fiberglass as the sound-moderating medium. A 216 with a split manifold and a set of "Smitty" steelpacks would rattle windows a block away when it was wound up!
Jerry
