49 CFR 325.91: Exhaust systems.

A motor vehicle does not conform to the visual exhaust system inspection requirements, 40 CFR 202.22, of the Interstate Motor Carrier Noise Emission Standards, if inspection of the exhaust system of the motor vehicle discloses that the system—

(a) Has a defect which adversely affects sound reduction, such as exhaust gas leaks or alteration or deterioration of muffler elements, (small traces of soot on flexible exhaust pipe sections shall not constitute a violation of this subpart);

(b) Is not equipped with either a muffler or other noise dissipative device; or

(c) Is equipped with a cut-out, by-pass, or similar device, unless such device is designed as an exhaust gas driven cargo unloading system.