Contrary to popular belief, motorcycles are not manufactured to be loud. When motorcycles come from the factory, they are reasonably quiet because they are factory-equipped with effective mufflers.
All highway and off-road motorcycles manufactured since 1983 are regulated by federal law under the authority of the Noise Control Act of 1972, which is still in effect. Federal regulation 40 CFR 205, parts D&E regulates the total noise emissions of both highway and off-road motorcycles. It also regulates their mufflers and requires motorcycles to be equipped with EPA-approved mufflers, not just when leaving the factory, but at "while they are in use." It is a violation of federal law to tamper with the emission control devices of motorcycles even those devices that control their noise emissions (their mufflers).
Excessively loud motorcycles are the result of the deliberate action of their owners who alter their exhaust system which is illegal. The most common form of illegal tampering is removing factory equipped EPA-approved mufflers and replacing them with non EPA-approved after-market mufflers or entire exhaust systems that are "for use on closed courses only" and not legal for use on highway motorcycles. Non EPA-approved after-market exhaust mufflers are not designed with effective exhaust noise suppression as a primary design goal. They are designed to increase the exhaust noise of motorcycles and their manufactures openly admit they are intended "for use on closed courses only." In other words, they admit their products are not legal for use on highway motorcycles. It is the unlawful use of those illegal for highway use exhaust systems that is the root cause of excessively loud motorcycles.