Sounds like an exhaust harmonic or when one of those tricked out Hondas with the big tailpipe gets resonant. Happens after warm up through all gears right at 2050 RPMs and only when in gear and accelerating. I can set the car to 2050 with cruise control and it will stay in this state as long as there is strain on the engine as in going uphill. seems to quit when going downhill or not under strain. I can't hear it outside but inside it sounds like it is between the seats. Not loud but noticeable.