We have been working on analysis of various muffler designs with CFD and find that the cross over X style like these seem to create drone and its probably due to the major pressure points on the overlap. We find similar flows in the X style Mufflers that are used too... streams collide together, so people add a valve to reduce the drone, but it seems like a flaw in an old school approach IMHO. Here are some visuals for anyone interested.
