They're dead quiet as long as they stay under 5 knots. Make them move and they're "loud and proud".
I have always wondered if that Chinese sub that surfaced behind the Kitty Hawk group in 2006 was being followed by a LA-class boat getting a sound profile on it.
All right, I’ll ask a question that will certainly display my ignorance, but why is displacement in a sub important? I understand why the noise it makes would make a difference but how much water it displaces puzzles me. Thanks.