Whenever the rockers or cam are done/replaced, the clearance between the rocker tip and valve stem can change, so part of the job is to use the special MB tool to measure the allowable thickness of the shims (the 'thrust washers' Cigar mentions below) which fit between the rocker tip and the valve stem. This is a very time consuming process. Are you sure it was done? The noise you describe could also be a lifter, plugged with a bit of dirt in the oil, easily checked and repaired. Try running some good top-end cleaner in the oil, change the oil and filter, to see if that doesn't clear it up. Do a search too, as there are many posts and decriptions of this problem on this forum.