I just went through soomething similar on a 340 a guy brought in and found the caps out of the hole behind the cam thrust plate, it was driving us nuts because it was only one lifter and just at 800 or so idle then it would go away with reving it a bit. It had 12-15 PSI at idle and it has always been enough in other engines I have seen but this one just would not quit the ticking. The pressure would jump right on up to 70 with a few RPMs. After putting in the plugs it now holds 50PSI hot idle and zero ticking.