Hello I was driving down the interstate today and was traveling about 70 about to get off an exist and down ****ted to third and the rpms went up to about 3500 rpms and started making aloud sound from the engine, sounded like a lose exhaust, or a head gasket blown. What could be making this sound?

 

Spark plug fell out? I had that happen once many years ago driving my mother's car after she had it tuned up. Scared the heck out of me at the time.

The place to start is, as always, check for codes.