I've noticed a few times that the engine puts out a "wooo" sound on. Each time I've let off the gas and the sound goes away. I don't know if it is the ANC adjusting or what. It wasn't that loud, but definitely present.
Just adding a data point here. It isn't worth mentioning to the dealer. I think what folks are hearing on this thread is a lot louder.
Mine is the same. Happens about 2200 RPM. Go above or below and it goes away. Mine is not that bad and not always there. As you said I think some are much more noticeable.