HOW NOISY CHINESE SUBMARINE WAS EASILY EXPOSED BY JAPAN ?
 

Chinese submarine technology is a bit behind the west the Xia and Han class used 4 bladed screws so im told, using copied technology from Russia in the Project 877 Kilo's they managed to get the noise levels down with the Song and Yuan class.

The new Jin class are significantly quieter than the Han & Xia but these were first generation submarines they still have some way to go.