The mystery continues....
Today I picked up new idler and tensioner pulleys, along with a new belt for good measure. Once I got the old belt off, I ran the engine without the belt and the noise was alive and well! So that eliminates pulleys and accessories.
I got it back together with the new parts and ran it for a while as I once again hunted for the source. I made a few observations that I'm hoping will be helpful in figuring this thing out.
1: I'm not sure if it's related, but is this normal? I am new to these trucks, and it seems odd to me that these bolts are backed out so far.
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2: What is this, and could it make the noise I am hearing? I put a screwdriver to it and listened from the other end, and I was able to hear the noise. So either that's it, or it's close to the source. (EDIT: Starter? but that wouldn't make noise while running would it?)
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3: I noticed the sound comes and goes when I rock the truck body side to side. Here is a video from below while I was rocking the cab. I had it in Reverse, since that makes the sound loudest.
https://www.youtube.com/watch?v=-dE2MZJO5po
4: Another video from below while I idle back in reverse and then brake, which also affects the sound. I go back and forth a couple times. You'll hear a squeak-squeak when I hit the brake to stop. I confirmed it is not suspension or brake squeak because I rolled to the same speed in neutral and there were no squeaks. It only does that when in gear (reverse).
https://www.youtube.com/watch?v=o1FI4_6BuuA
What do y'all think?