I'm no expert, but I think the most likely is stealth technology. This could be profiles of components designed to reduce turbulence and thus reduce vibration,

I'm no expert, but I think the most likely is stealth technology. This could be profiles of components designed to reduce turbulence and thus reduce vibration, or structures that minimize reflection from sonar etc. Since military tech is a cat and mouse game, having access to stealth tech could put you ahead of your competitors in the arms race, until they design another level of protection that makes current tech obsolete.