Kumba has a distinctive roar, but it appears that the sound is caused by something that is missing rather than something that is added.  B&M has a characteristic track design seen below of a box “spine” with fins that hold the rails.  This box can echo the natural vibrations of the ride and is often filled with sand to dampen it to the customer’s desired level. My guess is what makes Kumba unique is that these boxes are probably empty to maximize the “roar”, and not minimize it in any way.