First off, all SLR's are pretty loud. You cannot compare a focal-plane shutter and a swinging mirror with a leaf shutter. Add a motor-drive and there is no way you can be in stealth mode.
I think Nikon was more interested in making robust, reliable, high-performance machines than quiet machines. My OM2 is quieter than my FE2, but the FE2 has a faster top shutter speed and sync speed. And the FE2 is pretty darn quiet.