Thats the sound it has started making when it is first started and idling. It occurs at about a 1 second rate and seems to match the serp belt rate. The sound diminishes to nothing after about 3 minutes. I've inspected the belt and don't see any wear though. However, the belt is original and has 102K miles on it. Ideas?

 

Belts can get noisy and not show any wear. Could also be a bad idler pulley. If you think it is in the belt drive, you need to loosen the belt and check the pulleys for any noise or roughness. If you don't find anything, give the belt a spritz of something like wd40 and see if the noise goes away. If it does, you probably need a new belt.

 

That's the sound my water pump made before opting for early retirement. Take your fan, and wiggle back and forth hard with the belt off. My belt has 190,000 miles on it, and doesn't make a sound, and that is after having antifreeze and everything on it.