Grand touring summer tires are likely the quietest tires. They have smoother tread since they don't have sipes (not needed since summer tires aren't for snow anyway)
If you are after strictly the quietest tire you might consider time based replacement. It seems to me that every tire gets louder with age. If you want to always have quiet tires a new tire is probably best. Consider more frequent replacement using a lower cost tire.
Grand touring summer tires are likely the quietest tires. They have smoother tread since they don't have sipes (not needed since summer tires aren't for snow anyway)