I have a 98 Camry 2.2 4cyl. that seems to have a rattle/knocking sound coming from under the valve cover at about 2000 RPM. I took the cover off and inspected the cam lobes and lifters and they seemed to be ok. Is there a valve adjustment on this engine or does anyone know of common problems in this area to look for? The motor runs smooth and has good oil pressure. Thanks for any help.

Yes, it needs a valve clearance adjustment.
Very easy for a pro to do, not so easy for a novice.