Valve train noise

What would make a clacking noise at a certain RPM? It starts above 1800rpms and stops at above 2800rpms. It idles quiet. It did this before. I changed the lifters and it wasn't noticeable. Now it started again. You rev it and it clatters till it goes to idle and it's quiet. Cruising down the road at 60 and it's not there. Any body I talked to has no idea. It doesn't hurt performance either. This is in a 340 with a MP 508 cam. I put Comp Cam lifters in it. It has Crane ductile iron adjustable rockers and Summit push rods. This is a hydraulic cam. It's driving me crazy. Any ideas?

sounds stramge. Here's where i would look:
1. Oil is the level correct. If its too high and getting aerated it can cause lifter noise. is the oil pressure decent? Do you have and oil pressure guage on it? if not get one on there pronto. If the lifters aren't getting oil neither are the bearings (boom!)
2. How are you setting the preload on the lifters? If you're trying a half turn try an eighth of a turn and see what happens.
3. Go back to number 1. Sounds like oil delivery problem.

If the niose was there before,it could be a piston problem.What type of pistons are you running and the clearance?Common nioses are piston slap,rocker clearance niose,wrist pin niose,pre-ignition/detonation,header gasket leaks,etc.You may have a lifter that is loosing its oil and not pumping up at times.What clearance are you running for the valve lash,Mrmopartech

Had a 340 with the same what sounds like the same problem. When it was torn down, found a piston skirt missing. Again, couldn't hear it at all RPM's.

Oil pressure is good. Have a mechanical gauge hooked up. Oil pressure is between 45 and 50 lbs. hot at cruising speed and 20 lbs. at idle. I have KB pistons with 2 and 1/2 thousands clearance in the cylinder. Changed the oil with no metal. New head gaskets. No leaks anywhere. Now the clicking stopped. It comes and goes. I am puzzled. Thanks for the trouble shooting ideas. It has to be something.

The valve adjustment is 1/2 turn. I have tried different adjustments and it did nothing.

You said you've added some adjustment before with no results..Have you actually measured the lifter preload on each lifter? This is the amount of travel down into the lifter, that the plunger has with no oil pressure. You set it with your rockers. (it's absically the opposite of "lash", which is the extra space you put into a solid lifter cam.) I'm thinking a few lifters have the wrong preload. That can make some noise. generally, "1/2 turn" is about .020". Preload is usually a little more thna that, but tolerances in seat and valve machining during a rebuild can make that a WIDE margin. I would pull the intake off, and using a set of wire feeler gages or a dial indicator, take a measurement of preload on every lifter. I'd aim for .040 or so, and record how many turns on each rocker. Then you'll know for future adjustments should you need them.