I have had experience with Nikon system for a while, used D3100, D7100, currently D600 and D3. From these I would think that D3100 and D7100 sound roughly same way, pretty quiet; D600 and D3 are significantly more loud cameras.

Real question is "how quiet" you want your camera be, and how much you are willing to pay for it, given your investment into DSLR system.

If you need to shoot inside music hall and require different lenses (good quality tele/wide), you will have to get a real mirrorless camera. I have shot in amateur theater during performance and it was OK noise-wise (D600, non-quiet mode).

If you need to snap occasional street portraits, you can get away with cheaper mirrorless with single lens, such as pocket-sized Fuji X100* or anything similar, which is relatively inexpensive. However, I have shot those with much more loud film F90 (sounds like a tank) and was able to survive.

If you need to do some "spy", concealed long-range shooting, then you don't need quiet camera, just long lens. Same goes for sports.

Each camera and camera system solves particular problem, and we need to know your problem more precisely in order to point in the right direction.