Yeah, it could definitely be a baffle or such loose in the muffler. Is there a reason you won't consider an aftermarket muffler?...or maybe that's what you're suggesting? I think many of us have just done the muffler replacement deal with a Magnaflow, Borla, or Flowmaster, and just keep the rear dual tailpipes. It's easy and cost effective to do this, and the results are great. I think that unless you go the whole route of headers, y-pipe, and bigger rear tailpipes, just doing the muffler swap to a better flowing muffler is a pretty good mod.