On several occasions I set valve clearances to specification but still found them noisy. When the engine was disassembled I found large recesses worn in the heads of the bolts of the valve clearance adjusters.
When a feeler gauge was inserted to measure the clearance the worn section off the bolt head was not taken into account. With a “.010” recess worn in the bolt head the effective clearances become “.030” and therefore noisy.
It is very difficult to see this wear unless the valve is removed from the engine. Machining the bolt on a valve facer fixed this problem.