Well I just had the exhaust inspected. Other than 2 places were I managed to grind almost through the pipe.. The exhaust checked out fine. I've driven the car several times since then and haven't heard the noise again. However, the trips have been more like 30 miles, not 200. The temp seems to be hovering around 85 degrees.

So I guess the question that begs to be answered is..

What other than the exhaust could have caused this?

A problem with the IP that only shows itself after 2 hours of driving at 65+MPH?