The Swedes sunk the USS Ronald Regan in war games using a sterling engine powered sub. It makes it hard to track since it doesn't have the traditional explosion noises of a diesel and it lacks the heat sink noise of water cooling a nuclear core.
Diesel subs don’t operate their engines while submerged unless they are recharging in safe waters, so there aren’t any ‘explosion noises’. They have batteries that are used for propulsion, and the Diesel engines are technically generators, as their propulsion system is electric - like a train.