2 days ago I noticed this terrible buzzing sound coming from the rear of the car as I started it up. I reved it up and it stopped after I got over 2000 RPM. Let off the gas and it would start again. I got out and went around to the back of the car and it was the passenger side that was vibrating and buzzing badly. I pushed the tip and the muffler body itself fairly hard with my shoe and it never stopped, so I didn't think anything was loose or hitting because that should have at least changed the pitch if not stopped it altogether. Yesterday i had a chance to drive it up on ramps and crawl underneath and take a good look. I checked all the bolts and they were tight. The hangers were not contacting any metal. I had my dad rev it up to just under 2000 RPM while I was back there. Sure enough there was the annoying buzz. As he held the RPMs there, I pushed and held onto everything I could exhaust related to no avail. I went down to the shop that installed them and the guy checked it out and said it was indeed broken baffles inside. This is only the 2nd time in nearly 25 years he has seen this with Flows(this is my 4th V8 Stang with Flows and I have NEVER had this happen...until now. I know I have racked up close to 200K miles combined on them). He had to call Flowmaster directly since they only ship them in pairs. They told him it was no problem to send a replacement(free of charge), but would take nearly a week.