Where does the sound seem to be coming from?
Perhaps a higher RPM in sport mode is making a muffler problem evident.
As for poor MPG, I'll second that opinion about poor mileage. The
difference is minimal. I ran three, full tank tests on each of sport mode
& Non-sport mode. Mileage didn't vary more than one mpg at best.
You're missing the fun if you don't use sport mode.