Question is the maximum silent speed, not the level of noise at a non-disclosed speed, at unknown speed.
There should be a 3d graph describing the sub noise by frequency, at different speeds and deep.
And the most problematic is the low frequency noises, those propagating to very big distances.
With the same emission power, the 100 Hz propagate 60 times longer distance than the 1000 Hz.
So the most critical is the shape of the sub, the vibrations generated by the hull itself, and sadly any rotating machine that fall into the range of 1-400 rpm.
It is not simply machining tolerance, that is small part, but long and painful balancing of every component way beyond the normal requirements ( the rotating machines usually balanced only for the critical frequencies, to survive the spin up/down ).
Interestingly, the merchant ships has high noise around 50 Hz , so the non-damped submarine could have similar noise maximum.
The capitation itself can not be a big issue, that can be controlled by deep, and if they push the frequency to higher than 1000 Hz then the they don't need to worry about the emission power.
I think the key should be the shape of the submarine, the shape of the propeller, and the balancing of the rotating machines.
By these a low frequency noise source can be heard for thousands of kilometres : O