The car has a part in each rocker arm called a hydraulic compensator, it's sort of like an automatic adjuster, or like a lifter in a pushrod engine. There is a basic adjustment that should be done if any "real" changes are made to the system, such as grinding the valves, I also recommend doing this check if a new cam is installed. It takes a dial indicator and a special tool holding fixture for the dial indicator to check the addjustment. There are different thickness spacers that are used to adjust the basic position. Since the noise comes and goes, I would assume one of them is going bad, hard part would be finding which one. Another possiblity would be the small cup-washer on top of the valve stem was not installed, but again I would assume this would be a constant noise, not coming and going.
You're sure the long sheetmetal oil tube was reinstalled correctly?