Auto Parts: 02 Nissan Frontier engine noise, nissan frontier, driven plate

Question
My 1.3l 4cyl. Frontier makes a loud grinding noise when I  start the engine. It does not happen every time unless the truck has been sitting for awhile. It also makes the same noise when driving with the AC on. I have 73,000 miles on the truck and figure it might be the water pump, but the truck shows no signs of over heating.

Answer
    The first part of the description would make me think starter drive, but that couldn't make noise when the truck was running.  If it also happens when you turn on the A/C, then I would guess that it's the bearing in the A/C clutch.  The thing to do is to take the belt (or belts) off and then check each pulley by turning it slowly while concentrating on what you feel through your fingers.  If there is a bad bearing in any of them, you will feel the roughness as you turn it.  With the A/C clutch, turn the part that the belt attaches to, then try turning the driven plate (the outermost part of the clutch).  Also have a good close look at the space between the two, it should be the same all the way around, and the surfaces inside should look smooth, mostly.  The water pump could be bad, but they generally will make a high-pitched squealing noise when they fail, and rarely fail without starting to leak first.  Even the Alternator could be at fault, the electrical load on the alternator goes up considerably when the A/C is engaged, and could make a marginal bearing sound off, although they generally make a higher-pitched noise than what you are describing.  Checking everything with the belts off should give you the answer.