Engine has a noise that occurs at valve rocker speed

It has been doing it for the last 4,000 miles. It sounds a lot like a tappet. At cold idle, it is quite loud. It gets quieter when the engine is completely warm. It gets louder as I first speed the engine up to take off. Then it gets quieter. If you just rev the engine up to about 1,500, it gets louder just at first, and then gets quieter. After driving for a short while, when the engine is hot, I cannot hear it at idle. As soon as I push the accelerator down a little to take off, it get louder. As the engine speeds up, it seems less loud, but I think that is because the pecks come closer together, and there is less time for whatever is making the noise to do so. It is very hard to hear at 2000 rpm, but that may be because of the other engine noise. I have had two other mechanics listen to it, but they have no answer.
I have replaced all the cam followers, checked all the push-rods for straightness, inspected all the rocker arms and the blocks that hold them down for clearance and wear. I bought two new blocks and the yoke that keeps them in alignment, as I thought I had discovered a worn place. I looked at the cam position sensor and replaced it about.015 further out to see if it might be the timing chain with a loose spot. None of these things has made a difference. Well, the new tappets may be slightly different, but the noise has not gone away. I have not checked the timing chain and gears, but, since the noise is at rocker speed, I thought this was unlikely.
1995 Dodge W2500, V10, 5sp, a/c, 145,000 miles, cold idle oil pressure 60#, warm idle, 40#, warm 3000 70#.