Here's another post from Greddy.
http://www.greddy.com/CHPnoisepolicies.html

Enforcement Guidelines

The only drivers who should be cited are those whose vehicles:

1) are not equipped with a muffler;
2) clearly emit an offensive, harsh, excessive noise, or,
3) have a clearly defective exhaust system (holes, leaks, etc.).

Clearing Citations

When clearing excessive noise citations issued by the CHP or allied agencies, personnel are to consider exhaust systems in compliance if they incorporate a reasonably effective muffler, do not emit an offensive, harsh, excessive noise, and appear to be in good repair.

Q3 Do I have to actually listen to a vehicle to cite it for a violation of either Section 27150 or 27151?

Yes. Drivers of vehicles should not be cited for violation of either Section 27150 or 27151 VC unless the officer has personally listened to the vehicle in operation. T'his can be either under actual driving conditions or with the vehicle stationary and the engine running. Even if the officer has inspected the exhaust system and does not see a muffler present, the officer should listen to the vehicle. The purpose of this is to be sure that the exhaust system does not incorporate internally baffled pipes or other components that meet the definition of a muffler. There are no specifications which state required configurations or minimum dimensions for mufflers. A vehicle that does not visually appear to be equipped with a muffler, but does not emit excessive noise, should be deemed to comply with Sections 27150 and 27151 VC.

Rob