some mufflers have a perforated "baffle" disk inside. A lot of people just tket these out to get a little less backpressure in the system.

If you notice the engine is louder and its bothersome, then its missing the disk. You can cut one yourself from a tin can lid. It should be a snug slip fit into the larger diameter end of the muffler Then ther should be 4 to 8 holes drilled (1/8 to 1/4 inch dia), plus the center hole for the bolt. Total area of the holes should be 1 1/2 times the area of the muffler outlet.

You can experiment with different baffles. You won't hurt the engine, but with insufficient holes the engine won't run well. You'll notice it immediately on trying full throttle if you haven't put enough holes in it.