I have a Stihl BG85 hand blower. It has been used for at least four seasons, maybe five. In the last few days, the sound has been getting sharp. I don't know that I can explain it any other way. Clearly, it is louder, but am not clear if the difference is in the exhaust or something else. The sound is not a knocking, so it does not sound "internal." Each season, I mow about 1,100 or 1,200 properties and use it for drive and walk cleanup after mowing. And, in the Fall, it is used heavily for leaf cleanup. This season, it has been used more than others, some days working through 3, 4, or maybe even 5 tanks of fuel (runs about 45-50 minutes on a tank). It runs fine, it starts fine, with no difference in performance. I am wondering if the muffler has developed a hole or extraneous passageway. I have checked and the muffler is not loose. I keep thinking the sound difference is exhaust related. During the mowing season, when it was used for mowing debris cleanup, the baffles on the exhaust opening of the muffler were black. However, now that it has been run hard recently, the baffles are clean. There is no sign of carbon blackening. Anybody have any ideas on what to check?