I bought Dick's "Petrel", and have about 3 hours on it with his ZDZ 40. Motor is hard mounted, prop is Bambula 20 x 10, muffler is his Vtech canister...it was pretty noisy until I soft mounted the entire exhaust system. Between running the Bambula (very quiet compared to others) and the soft mounts, airborne, this airplane is no louder than MANY, MANY current pattern designs.
Reason - in flight throttle settings are nearly ALWAYS less than 1/2 power that is available with this motor.
As someone else already said, it's almost TOO much power, but the throttle curve function has really helped. It's a pretty remarkable airplane.....see my website, click on "Petrel" to read about it and see photos.