take the cover off and see what falls out. could be the gears need grease or the cover is rubbing on the gears. One of my engines gears whine loudly at a certain rpm. check the small gears slotted screw and make sure its tight. The cover on some engines will resonate like a drum on some of these engines as well. That problem can be addressed by putting a sound deadening material on the cover if your hands quites it down when you lay your hand on it its possible the cover is making the noise. Check out the cover.