New batteries and HFC or other similar fuel cell technology is quiet, but no matter how the energy is created or stored it runs an electric motor to actually move the sub.
A Kilo class sub with a German hydrogen fuel cell and batteries will not be any quieter or louder than a Kilo class with batteries... because it will use the same electric motor for propulsion whether the power is coming from a fuel cell or a battery it is just as quiet.
Not all AIP systems are quiet but all extend the time a sub can operate underwater.
The new Megawatt NPP for powering lasers would be an excellent component to add to an existing SSK and could be used to charge batteries and operate the sub without needing to surface.
In fact they could easily fit 2-3 of them in a Kilo sub replacing the diesel engine and diesel fuel tanks that would no longer be needed... they could be used like the diesel engine... each of which generated 1,000Kw, so most of the time they would be off with the sub operating on batteries and then when the batteries need a charge... no need to surface and snorkel... just turn on the two or three NPPs to fully charge the batteries while remaining fully underwater... in fact when you are chasing something you could turn on the NPPs for an extra energy boost to increase speed and endurance...
Even if these NPPs were noisy when operating they wont be as noisy as two diesel engines and the fact that they don't need to surface makes them vastly more stealthy. You could drop them into the design like batteries themselves and swap them over just as easily.
As technology improves the power they generate will increase, their size and weight will decrease and the costs will become cheaper and cheaper.