After a lot of research I found posts talking about their being stress cracks around the spot welds that hold the flange that the door hinge bolts to (seems like it's a large torx bolt on most 850s). This is where the hinge 'arm' bolts to the car frame on the front pillar.
Well, it turns out my 97 850 had the fatigue crack on the driver's door itself. It was impossible to see without using a mirror to view the nuts that secure the door hinge to the interior of the door. I first thought the popping was the hinge itself, but inspection showed no issues. Then after reading about stress cracks in the sheetmetal, my mechanic used a mirror and, lo and behold, there was a 1/2 to 1" stress crack from right below the upper door hinge nut, running up past the side of the nut head. Sorry, I could not get a camera in there for a pic, but if you have a popping or clicking sound in your door hinge you can't find the source of, look for very small stress cracks in the sheet metal on either the door itself or the front pillar of the car where the hinge arm connects.
Luckily my mechanic was able to tack-weld the crack (after sanding off the paint over the crack), then we put in an extra large washer under the nut to support the metal around the hinge that flexes when the door is fully extended. When I take the door panel off next I will also take out the hinge and put some type of thin reinforcing plate on the back of the metal as well to hopefully avoid breaking the tack-welds in the future.
Obviously this is a design flaw - Volvo should have put in a reinforcing plate where the hinge bolts to the door, or heavier sheet metal. But at least if you know what to look for it's way easier to fix.
I hope this is helpful. I bought this 850 used about 5 months ago and had never run into this issue on any other of my 7 Volvos, so it was quite a relief to get rid of that darn clicking noise. Now on to other issues.
