The U.S. Navy has revealed that the Chinese Navy has turned its attention to making its submarines quieter. For decades, the Chinese concentrated on just building subs (no easy feat, as few nations can do it) that were reliable enough for wartime use. In the last decade, China has sought to make its subs safe for peacetime use. There have been several bad failures of Chinese subs. In one recent case, the entire crew of one boat was asphyxiated when the diesel engines did not shut down as the sub dived. There have been numerous breakdowns while at sea, and many subs that don't leave port much because of reliability problems.
Diesel electric subs are intrinsically very quiet when underwater, operating on battery power. But the Chinese did not train their crews to be quiet when "running silent." This included tweaking the mechanical items, that run off battery power underwater, to be quiet. Thus U.S. ships, and especially nuclear subs, had an easy time detecting Chinese subs, even the diesel-electric ones running underwater.
This is all changing. Chinese dockyard workers and engineers are silencing noise making components. Crews are trained to operate silently when the ship is running under water. New nuclear boats are also being refurbished to increase quietness.
Despite all this, the U.S. Navy has found that Chinese subs are still noisier than Russian boats were 20-30 years ago. But if past performance is any guide, in 10-20 years, Chinese subs will be very quiet, and much more dangerous. China is in the process of expanding its sub fleet from about 60 boats to, over the next decade, 75 more modern ones.