Back in my mustang days, my understanding is drone is caused by the 2 mufflers being the same ans they are producing the same note that are just slightly outta tune from each other. Im sure other v8 owners have noticed driving by another v8 car the notes start to modulate as you get near each other they eventually match and stop as you keep going. I know some mustang guys actually used 2 different size or style mufflers and it did work