Here is what I found...
The exhaust pipes in automobile exhaust systems
are designed as acoustic resonators that work with the muffler
to reduce noise, by making sound waves "cancel each other out"
. The "exhaust note" is an important feature for many vehicle owners, so both the original manufacturers and the after-market suppliers
use the resonator to enhance the sound. In 'tuned exhaust' systems designed for performance, the resonance of the exhaust pipes is also used to 'pull' the combustion products out of the combustion chamber quicker.