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?