what is the cause of a clicking sound inside valve cover?

Probably a sticking or loose rocker arm.  You can try a quick fix by adding some Marvel Mystery Oil to the engine at your next oil change, but you may need to remove the valve cover and tighten the rocker arms gradually until you find the one that is sticking.  My parents 1992 Jeep Cherokee developed this same problem as it aged, but it has not caused any major issues for them, and we have eliminated it by tightening the rocker arm nuts in the past.