[mndriver]

Don't go replacing it too quickly.

I learned the baffles inside like to build up with soot/carbon. Tap on the muffler with a hammer and watch the debris come out of the muffler.

And listen to the APU suddenly get quiet again.

[Roadhog]

Just went though cleaning out the carbon buildup on my ATV muffler.
They make the muffler with a plug you remove, to make removing the carbon easy, but those plugs usually have to be heated up red hot, before they budge.