Bigus Sub Noises
Nuc subs have lots of pumps etc running, the smallest error in soundproofing allows sound to escape and sub to be detected, identified
Diesel subs running on batts have only slick electric motors running so no pumpy things
Hardest subs to find are some Dutch which started life as nuc so had lots of soundproofing but government changed so they were no longer acceptable and during build only diesel elect installed so have lots of soundproofing on electric motors silence