I did another test using the sound meter, not letting go of the shutter release this time. It recorded 58db. Therefore, it is just about 1db quieter than the normal mode, not a big deal. : )
I bought my D610 two weeks ago and have taken nearly 900 photos so far. While I agree that it's a fine little camera and the ideal travel kit paired with the 24-85VR, I am not convinced that the quiet mode is quieter. I have a sound meter. Pointing the lens close to the tip of the sound meter with VR switched off, it recorded a peak of 59db in regular mode and 61db in quiet mode. I think it is loudest when the mirror is lowered.
What a stunning shot!