Diesel boats, at least engine wise, are far quieter than nucs because submerged they run on their battery, An electric propulsion system produces virtually no machine noise, except the screws, and technology has provided for very quiet propellors.
ASIDE: one of the secrets sent by John Walker to the Soviets was the make up of American subs’ props and screws. The Reds got the Swedes to machine copies and produces what US submariners called the “Walker” class boats.
Why did the Soviets need the Swedes to do that????I presume they had adequate tech themselves if you are talking about equipment.Besides,wasn’t Sweden wary of the Soviets??