It's possible to fine tune the noise level and back pressure by plugging holes with 4/40 screws. I can't say that I've needed to do that, but I have seen it done. I suspect that a motor that was running away might be regulated by plugging holes.

All my OS 20s and 40s meet the 90dbA rule with tongue mufflers and aren't subjectively very noisy. We measure and log noise levels as part of our deal with the city for the flying site.