The sound is not sustained. It only makes the sound when the engine is thrown into a heavy sudden load. For example, when coasting forward, and then suddenly throwing the engine into reverse at cruising RPMs. In this case, once the boat slows, and gradually starts moving backwards, the sound stops.
The way I discovered this was when I use reverse to slow the boat--like when coming into dock. I went and checked things out on my lunch break. it appears that I may have a little too much gear oil in the gear box. I'm going to verify this, but I wouldn't think this would cause the sound.