Neb. Rev. Stat. § 60-6,286: Muffler or noise-suppressing system; prevention of fumes and smoke; requirements.

Every vehicle shall be equipped, maintained, and operated so as to prevent excessive or unusual noise. No person shall drive a motor vehicle on a highway unless such motor vehicle is equipped with a muffler or other effective noise-suppressing system in good working order and in constant operation. It shall be unlawful to use a muffler cutout, bypass, or similar device on any motor vehicle upon a highway.

The engine and power mechanism of every motor vehicle shall be so equipped and adjusted as to prevent the escape of excessive fumes or smoke.