Swedes make good subs. There is the story about the US Navy leased a Swedish sub with AIP for about a year. The sub wrecked havoc on the US navy during exercises off San Diego.
I guarantee that the U.S. Navy learned a lot from their time with the Swedish sub, and they have put what they learned to good use. Nothing with machinery is 100% quiet. The noise is there, you just need to know how to detect it.