I have a 2005 Honda Accord Hybrid (bought Nov 04) with just over 10,000 miles on it and have also been experiencing a loud, low frequency rumble when the engine is at about 1600-1700 RPMs in ECO mode (3 cylinder operation). I suspect that when in 3 cylinder operation at low RPMs that the resonant frequency of a major component of the car is reached and that the sound dampening becomes ineffective. Or, perhaps there is something wrong with the active noise cancellation technology or the active engine mounts. I haven't yet had a dealer look at the issue.