Check around that valve carefully for a broken valve spring or something. It is very strange to find a loose clearance in the area the mechanic suggested the noise was in.

Especially if you do not feel you did the valve adjustment wrong so short a time ago. Could the piston have been tapping the valve back up for example? Try to see if you can pull or pry that valve higher to reduce the clearance you feel. Do not be too aggressive though.

Or press the valve down and see if it is returning properly as well. Of course the engine will have to be off top dead centre for that. It will probably have to wait until the item you are working on now is completed. I only assume you put the engine at top dead centre before you started.