how can I fix my opel corsa 130is 1998?

My car makes a noise from the engine like marbles,the mechanic told me its camfollowers then I changed it.Another mechanic told me its a bearing knock then the last mechanic told me its my shims when this car don't take shims,Please help me understand this vehicle as I want 2 sort this problem.I'm driving my car with this noise for the past 2 years so it can't be bearing knock if I can just get this noise 2 stop even tried the best oils nothing worked.