This is a world class lens. Yours needs work.
No longer. I fixed it. Piece of cake.
That's good news. My 105/2.5 is indeed one of my favorite portrait lenses. Love the character.
Great to hear you fixed it. BTW was it suffering sharpness wide open at specific focusing distances? The rear cell must have totally came loose for significant rattling to be heard. Additionally, and depending on lens design, a slight shift of the rear group away from the front group usually just results in a nominal change of focal length that tends to be compensated for when you check focus through the viewfinder. This is why it's not strictly important that the tightening of the real group isn't exactly as it was at the factory - as focusing shifts the front away from the rear anyway. That's why I ask if this was only present at the limits of focus or if the rear group was completely unthreaded. If its the latter I'm surprised you got sharp images at f8.
Well to answer some questions, what I did was photograph a gallon plastic weed killer bottle that was about 45 feet away at the bottom of the yard. The bottle was a good subject as it has writing on it. My initial tests showed it to be a bit fuzzy at full aperture, with improvement with the lens shut down. On re-reviewing, it looks like the ground immediately behind it was sharper, leading me to wonder if the rattling rear element caused some erraticness in the precise plain of focus. Now the image is much sharper and compares with images shot with other Nikon lenses that I have (80-200 f4 and 35-135 f3.5/4.5). With none of the lenses is the small writing on the bottle legible, but the images with each lens are about as sharp as each other. Again, the test was done on an oldish digital camera (D200) which also introduces some limitations. My 35-135 produces razor sharp analog 8X10s so I am sure the 105 will now too.
The rattle was not significant, and the tightening involved an almost imperceptible turn of the element, but it makes quite a difference.
I'm surprised that an almost imperceptible turn of the rear element previously had audible rattling honestly. I'm also surprised that what you saw in the viewfinder, focus wise, resulted in back focus in the final image but then again this was 45' away. Anyway I'm glad you got it fixed.
That rear set of element may just need a bit of torque. Lots of old ones do.