Tripac APU muffler getting loud?
 

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.

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.