A nuke sub runs soundless and undetectable at 30 kts (nearly 35 mph)? At what speed does the nuke sub become detectable? I have limited understanding of sound attenuation on nuke subs...for a number of years I was involved in development & manufacture of a product for the nuke subs that was designed to prevent the transfer of all vibrational "sound" to the hull during operation. Then our subs were quieter and less detectable than anything the Soviets had in their fleet. This is likely still true.
The sub may not be soundless, but there are techniques, which I am not at liberty to discuss on a public forum (Loose lips sink ships is literally the case), that can be used to keep the sub from being detected.