In most cases, the angle of the short turn radius of the exhaust port creates the sound difference -- that is if all other components are equal (cubes, cam, compression, etc). The speed of the exhaust as it rolls over the short turn and gains speed from the valve makes most of the noise in an engine. This is a sonic boom that is relative to the speed of the moving air. Thus, a high compression engine will sound louder, even at an idle. The same is true with a supercharged engine of the same compression.