Once my engine warms up (~1,900 miles) the machine develops a "whistle" at a low rpm. The whistle passes when you get above and below the specific rpm range, which is a little above idle. Imagine this; no whistle idling, whistle while taking off, no whistle while more than a few hundred rpm above engagement, then a whistle while the rpms come back down to idle. My high tech echo-location test (moving my ear all around while revving it up and down in neutral) indicates that it could be the valves based on location. Does that make any sense?