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 viewing

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.