We just fixed an ST90 that was making a metallic sound. It was heard best at idle. Sounded like somebody tapping on the muffler with a screwdriver. Turned out to be the stock muffler. Must have had a baffle loose. It also had trouble keeping a throttle setting which we later traced back to insuficient fuel draw. Put on another stock muffler from his other ST 90. Problem seems to be gone. He's had several good flights without dead stickin and doesnt get any waivering in the engine while flying.
Edwin