As others...not sure EXACTLY what you're hearing but...
If you're depleting the battery range or running in HOLD or Mountain the engine will rev up as needed to replenish the battery to the reserve or back to where you set it at. There is quite a disconnect between what your foot is telling the car to do and what the engine is doing. This is why many use the ICE in highway or non-residential areas and save some charge for more quiet areas and switch back to battery in those instances. It doesn't make as much of a racket if you put on hold with some charge on the meter (I like to set at about 5 miles and then tap that as I approach home) as opposed to a depleted charge and the generator trying to replace depleted reserve.
Again, not sure if this is what you're hearing.
If you're depleting the battery range or running in HOLD or Mountain the engine will rev up as needed to replenish the battery to the reserve or back to where you set it at. There is quite a disconnect between what your foot is telling the car to do and what the engine is doing. This is why many use the ICE in highway or non-residential areas and save some charge for more quiet areas and switch back to battery in those instances. It doesn't make as much of a racket if you put on hold with some charge on the meter (I like to set at about 5 miles and then tap that as I approach home) as opposed to a depleted charge and the generator trying to replace depleted reserve.
Again, not sure if this is what you're hearing.
