I was at the lake last weekend and noticed a clanking noise coming from the rear of the engine compartment when I opened the engine cover. Sounded like it was coming from the serpentine belt/pulley area. Decided I should cut the day short to avoid getting stranded on the lake. Took the boat to dealer and they couldn't find the noise until it warmed up. Then they found noise coming from inside the muffler. They say there is a flap in there that has come loose, and this requires muffler replacement. Anybody else ever experience this? Technician says it's unusual. Curious what could cause this?