And you guys are sure it is not the cylinder deactivation and MDS kicking in? Have you de-activated that and seen any change in the 'droaning' noise? When my MDS kicks in (Green eco light goes on), I think there is a belt activation of sorts (read about etourqe and this is one of its 'features') that kicks in and then the cylinders drop. My MPGs go up and I just idle foot it. Spitballing here, but just seeing if it is actually be design.