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?