Definitely not the fuel pump. One of the rockers was out of adjustment - I think it was my fault. Got that tightened up. I bought a bore scope and looked down in #4. Even though it's obviously seen a lot of miles, it's fine. Nothing amiss.
So, I put the valve cover back on, got it all back together, and thought maybe I could have "fixed it". No.
So, it does not sound, nor ACT like a rod knock. It's very different. It only happens when the engine has no load on it, at around 2k rpms. It does not do it when the engine is under load going up a hill, nor when going down the hill and foot is off the gas.
But if I'm cruising around 50mph, LOUD.
My neighbor stopped me and said, "Dude, that's coming from under the truck, in the middle!"
So, he helped me out, and I got under there with the stethoscope. The sound is even louder at the bell housing. I am not sure if it's the throwout bearing, the input shaft bearing, or something else that came loose, but it seems to be coming from there - even louder than the fuel pump area - which seemed obvious to me.
So, I am going to keep looking at this, in hopes that I can get this fixed in short order. Luckily, the truck can still drive, but I am avoiding that since I don't want to cause more damage.
