Mont. Code Ann. § 61-9-435
Exhaust noise limitation
61-9-435 . Exhaust noise limitation. (1) Except as provided in subsection (2), a person may not operate a motor vehicle with an exhaust system that emits a noise in excess of 95 decibels, as measured by the society of automotive engineers' standard j1169 (May 1998).
(2) This section does not apply to a motorcycle or quadricycle that is subject to 61-9-418 .