I had one that did the same thing some years ago when I switched cams and you would not hear the lifter at low RPM's but after you revved it, you would start to hear them chatter. I had used the same old push rods and they were just a tad to short because of the new higher lift cam. So I got some new longer push rods and the problem went away. Had a buddy come over with some kind of ajustable push rod to measure the right length. The old ones were about 1/4 of a inch off. Kurt