Yesterday afternoon, I started the truck up to quiet my two year old... And I had some "chatter" in the fan belt. 223 i6.
On the first day of my vacation, the "honey do" went out so I decided to investigate.
This morning it started up with no noise. I drove the boy to school and it started to chatter. I had an old can of belt dressing so I sprayed the belt and it quieted down a little.
I got home, the belt felt a little loose, so I tightened it a little.
It rained over the weekend hard and it was very moist outside with mist and fog. I decided to spray the bearings on the alternator and the idle wheel on the bottom with silicone spray. The sound was significantly reduced and in fact, got really smooth at all RPMS.
Just went to lunch and it sounded fine. Drove 20 miles, nothing. Got home the noise was a little loud at the stop sign but no noise in my driveway. Shut off truck and restarted the noise came back.
New belt, about 3000 miles on it. New alternator. Everything lined up well. Alternator is tight.
The only thing I can think is the idle bearing? or another issue I have is a leaky valve cover gasket. It has gotten a little worse than a few drips and it is right down the front of the engine so I am changing that this afternoon if Napa gets it in before dark.
While you're working on the front of the engine, pull the belt and turn all pullys by hand. The belt noise could be a byproduct of something else going out. You will probably be able to feel a roughness in one of the items. Also check the water pump for looseness. Grab the fan (the belt IS still off and the engine not running, Right?) and see if you can wiggle the WP shaft in the housing. This can usually be felt better if you hold the ends of the blade. Don't think that just because the alternator is fairly new that it couldn't have dry bearings. You could even take it to an autoparts store and have them test it. It may be noisy on their machine, too. There aren't too many options on the six. I'm sure you'll find it. I hope you find it at home and not on the highway....
Nothing is loose. The belt initially might have been loose, I think water and oil dripped onto one of the pulleys. It's been driving smooth. If it starts again tomorrow morning, I'll pull the belt and get more thorough.
Spurrdon brought up an interesting point. Is this a home made conversion or did you buy one? Does it use the old style V belt or a multi ribbed belt? On any conversion, alignment can be a real issue. Try to get a real good look at how things line up.
I've had some issues with chirping noise on my car with a multy ribbed belt. Tried belt dressing(never again, messy). What I do to get a new belt to seat if it's making noise is take a wire brush it. One of those small ones. About twice the size of a tooth brush. Lightly touch the ribbed side and see if the noise changes. If it doesen't, try the back side. That will let you know where the noise may be comming from. The back side only touches the idler and tension pulley. Good luck.