One thing that I neglected for too long was the "Dryer" or Drier" filter/rebuild kit....its the thingy that does the hissing sound and keeps your air system from getting moisture in the system.
I generally change my Fan belt every few years but my access on my Cat is relatively easy(side radiator). I check my hoses whenever I have the engine cover open. One thing I recently came across was a large tear in one of the Turbo Charger boots....which can cause all sorts of problems so do check all the connecting boots on your Turbo(access from under bed, I'm guessing).