On my left door (driver's side US), the spot welds that hold the tab for mounting the aft window channel had both failed. (Perhaps this contributed to the wind noise I've also experienced, perfectly described by @Thom in this post about wind noise.)
Once the outer spot weld fails, it leaves a telltale crack in the door skin. I had covered it with touch-up paint, thinking it was a very odd chip.
This YouTube video exactly illustrates the problem.
My right door (passenger US) had one broken spot weld (the inner one). I clamped them up on both doors and zapped them with the MIG.
