RCW 46.37.390
Mufflers required — Smoke and air contaminant standards — Definitions — Penalty, exception.
(1) Every motor vehicle shall at all times be equipped with a muffler in good working order and in constant operation to prevent excessive or unusual noise, and no person shall use a muffler cut-out, bypass, or similar device upon a motor vehicle on a highway.
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(3) No person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the engine of such vehicle above that emitted by the muffler originally installed on the vehicle, and it shall be unlawful for any person to operate a motor vehicle not equipped as required by this subsection, or which has been amplified as prohibited by this subsection. A court may dismiss an infraction notice for a violation of this subsection if there is reasonable grounds to believe that the vehicle was not operated in violation of this subsection.
This subsection (3) does not apply to vehicles twenty-five or more years old or to passenger vehicles being operated off the highways in an organized racing or competitive event conducted by a recognized sanctioning body.