[CoffeeDregs]

I think you're referring to "Jake Braking", which is very noisy and is banned in many towns (in California).

https://en.wikipedia.org/wiki/Compression_release_engine_bra...

[Unklejoe]

"Jake Braking" is similar, but pertains more to diesel engines which have a special valve to support this since they work differently.

Engine braking on a gasoline engine is actually quieter than normal operation.

[mmagin]

At least the manufacturers of those products assert that correctly installed ones are no louder than the diesel engine in normal operation.

http://www.jacobsvehiclesystems.com/about-us/environmental-h...