Strange Rattle in Right Rear Wheel Area

Anyone else hear of this problem...there is a really strange rattle, somewhat loud, that's coming from my right rear wheel area. It only starts at highway speeds, is very intermittent and starts and stops for no apparent reason. The only common element when it does occur is that increased vibration sometimes changes the tone and frequency. At first I thought there was something wrong with the Flowmaster muffler I had installed so I had it replaced with the same thing but to no avail. It seems like it's some kind of strange exhaust problem but others have slapped in a Flowmaster muffler with no problem. Any advice on what I should do?

Hey pwhite05....I wonder if we're hearing the same thing? I posted this exact question on the other Stang site and got a reply that sounds more likely. The reply came from a guy who had an after-market muffler installed and it turned out to be that the brackets were rattling against the muffler as the exhaust system heated up and expanded, every so slightly, the muffler itself. He moved the brackets and the problem disappeared. I'm hoping that's the solution for me but I'm not holding my breath. Do you have an aftermarket muffler or stock? As for the sound, the best I can describe it is that it almost (repeat; almost) sounds like that bad-gas or "car needs a tune-up" rattling engine knock...but more metallic and tinny. I'll poke around my recently installed Flowmaster and see what I see.

As for other sounds and glitches in the 05 although some say it's because Ford hasn't worked out some bugs I hate to say I don't believe that. Don't get me wrong, I absolutely love my 05 V6 but let's face it, the Mustang (V6 or GT) never was nor will it ever match something like a Jag or Porsche or Mercedes in terms of quality components, fit, finish, etc. There's a reason the Stang is a $20-$30,000 car and not a $50,000 + vehicle.

I FOUND IT!!! Finally, after two months of living in rattle hell and the guys in the muffler/exhaust shop treating me like some kind of retard, I found the source of my rattle. It is (was) indeed related to the installation of my Flowmaster; all it was was a single steel rod brace that was welded to the back of the muffler ("back" in the case referring to end of the muffler closer to the front of the car) that hooked into a bracket on the frame. Surprisingly, when the car/exhaust is cold, there's about a 1/8 inch gap between the rod and the bracket. However, when the exhaust system heats up, everything expands and pushes that rod right up to the bracket where vibration from the moving vehicle causes it to rattle. That seemingly tiny contact point resonated through the muffler and through the entire right rear wheel area. I can't wait to tell the "I've been doing exhaust for 15 years.." unbeliever at the muffler show that metal expands when it's heated...potentially causing problems you wouldn't consider when the system was cold.