I have a Challenger, 469 ci, 3" inch all the way out the back with a DR.Gas X- pipe and Dynomax Turbo if that matters. No drone. From what others indicate, perhaps it is those mufflers. However, it's hard to tell since no one has the exact same combination/setup as you.

Is it possible that your X-pipe is the issue. I'm no exhaust expert but maybe you are experiencing some problem due to the design or lack of due to the X-pipe. Supposedly they are not all created equally

One other thought and I'm not trying to be a wise azz, but are those mufflers designed to flow in one direction and are the installed properly?

Just a thought...