Can you post a video with it making the noise in question? Sounds like turbo noise to me based on your description but it's hard to define a noise on paper, even virtual paper.

Turbos will make a sort of grinding noise, especially on deceleration as it spools down. What it's doing is the impeller is actually "cutting" the air and there's little exhaust noise to cover it. Truck (the Dodge/Cummins) is hard to hear because it's a noisy engine but Mutt (the Chevy/Mercedes) is pronounced even with no exhaust. I expect the new, big turbo will be even louder - I'll let you know.

Dan