Pro tip: the old timer at the counter in Boise taught me to listen to a shutter before I ever open the camera
I was dropping off a box of mail at a little strip mall in Boise when I saw a repair guy sitting outside his door with a Nikon FE, just firing the shutter over and over next to his ear. I asked what he was doing and he said, "I figure out where the trouble is before I take a single screw out." That got stuck in my head for days. I used to pull the bottom plate off right away and start poking around, and half the time I would find nothing and put it back together no smarter than before. Now I sit with the body, fire it at every speed, watch the mirror, and just listen. A drag at 1/125 that sounds fine at 1/500 tells you way more than a parts diagram ever will. Took me two weeks of doing this on junk bodies from a flea market lot before it clicked. Do you folks diagnose by ear first, or is that just me being superstitious?