Do they sound different than a typical Cat/Cummins with the brake on ? As bad as they cackle with the throttle applied, I could imagine quite a racket while engine braking.
Appears to depend on the muffler quality. As a favor I drove a Freightliner cattle truck for a few months out of Or. down to northern Ca. It was very quiet for a 92 series. The old IH lo-boy was about like a Cummins noise wise. My neighbor's Pete logging truck with the 8V71 was absolutely vulgar. It either had straight stacks or rotted out mufflers, can't remember which. It was bad enough under throttle but it would wake the dead when you let the brake come on. The thrill wore off after about the third day I drove it----earplugs were mandatory. Another driver later rolled and totaled it so in later years the more restrictive noise limits were of no concern.
Just remembered, before retiring, our shop maintained about 5 brand new KW's with Silver 8V92's + Jake brakes. With the modern exhaust systems (at the time) those engines were amazingly whisper quiet, Jake and all.
Just accidently found this on Google, good electrical schematic, Jake Brake cutaway pics etc, for 71 series http://bernhardbus.com/JAKEBRAKE.html