I had the same situation with a new original 126 muffler and all new 2.5" piping on a 617a. I added a small resonater early in the piping (down-pipe), and the problem was solved.
Alan has the right idea - muffler location and overall pipe length can be manipulated to eliminate the resonance, but it would be tough without good software or lots of guesswork. My resonator was an off-the shelf Toyota part
The noise is gone though!