It isn't a true LB7 rumble if its constantly changing My tuner generally makes tune 1 & 5 have LB7 Rumble. When its idling the vanes stay right at 3% on those.
Has to be one of the most annoying noises I have ever heard, especially inside the cab. Fortunately my rumble tune is only on the "mileage" tune and I rarely if ever use that, just need the go fast tune....