Try adjusting the valves. And when you do, be shure to check for any broken or loose parts. I found that one of the wavy spring washers was broken. This caused the rocker to move back and forth on the shaft which hammerd the adjustment screw and made it uneven, hence throwing the adjustment off and causing the valve tap. It was easy to fix. the push rod was not damaged so I only had to replace the wavy washer, ajusment screw, and clips. I did this back in August and it's been heaven since. Don't forget to adjust the valves afterwards. You have to remove the whole rocker assembly to do it. I used wheel bearing grease to hold the push rods in place during reassembly. Make shure the push rods are seated firmly in the lifters. It takes a little patience.
Good luck.  John 90 vanagon gl