I second that - I hear/feel the same in eco mode...I notice it when in ECO mode.
My theory is you are at the point of almost but not quite ready to downshift. You typically don't notice it because the avg cruise speeds are well past this point at all gears.
So for instance if the computer says the shift point based on your driving style is at 2500RPMs and you are at 2510RPMs you might feel that rumble persay because the computer is waiting for the drop in RPMs to downshift.
Yes I know 2500RPMs is high.