JMO, pull the valve cover and pull the rocker Assy. Take the pushrods and measure to see if they are all the same. If not then correct the issue. If they are the same manually manipulate each valve using a pry bar to make sure each valve moves/closes smoothly. If they don't then soak with penetrating oil and work until they do. If that doesn't work then the head will have to come off. If they all work smoothly then reinstall everything and properly adjust the valves. Hope it works out.
