Drone is caused by two different, but very close frequencies (pitches) interacting with each other. Two functional outlets on the same muffler, but different lengths from the inlet of the muffler will "speak" at different pitches. Drone. A twin engine aircraft or boat that doesn't have the engines running at exactly the same RPM will drone. If the head pipes are slightly different length, it will drone.
Look up "pipe organ tuning" on youtube. Pipe organs are tuned by sounding the pipe being tuned at the same time as one that has already been tuned. The tech listens for the "beat" of the sound. He taps the pipe's tuning collar to change its length, and the beat slows as it comes closer to tune. When perfectly tuned, there is no beat. My guess is that production exhaust systems are carefully designed and produced to make the system speak with no mismatch in frequency. Aftermarket systems, and muffler-shop solutions probably never even consider what a length mis-match does to the sound.
I feel this "drone" thing is a simple matter of the two mufflers or outlets speaking at different, but very close to each other, frequencies.
I suppose one could fit tuning collars to the outlets, and tune them to match each other with no beat. It works for a pipe organ, it would work for an exhaust system too.
