As I said, 95 decibels seems like a reasonable enough limit for motorcycles. But I suspect lower limit for motorcycles is motivated by people getting fed up with motorcyclists particularly. The worst motorcyclists have earned a horrible reputation for all motorcyclists, I think this lower limit is a consequence of that.

I think it was just because they generally made less noise when the regulation was enacted in 1972. Car design has changed a lot since then to make them a lot quieter. Bikes have changed relatively little.

If movies are a gauge of public interest, then people have been upset with motorcyclists since the 60s: https://en.wikipedia.org/wiki/List_of_biker_films

I don’t think Hollywood stereotypes of biker gangs was a predominant motivator for the Noise Control Act of ‘72.

This is a regulation that generally regulates noise for all kinds of things, from appliances to airplanes.

Their stated rationale is: https://www.law.cornell.edu/uscode/text/42/4901

I think Hollywood wasn't motivating anti-biker attitudes, just the opposite. Hollywood made these movies because the subject was of interest to the public. Motorcyclists were already notoriously antisocial by 1972, and that public sentiment probably informed the Noise Control Act of '72.