I'm still not convinced by this or either of the other replies. Safety issues I should be protected from by the government. "It's loud" is something I should walk away from after my lease is up without the government ever being involved. Some people appreciate cheaper options even if they are louder and there isn't a good reason to have the government prevent them from being allowed to take those options.
I wouldn't object to some kind of noise rating system instead. The point is just that soundproofing is not something people can judge effectively during viewings, so it doesn't get priced effectively by the free market. Maybe a noisier property will have slightly higher turnover, but the rental price will probably not be noticeably affected.
