Common for holes in oil tube to clog with sludge/varnish over time which first evidences itself by starving cylinder #4 camshaft lobes for oil. Even if this is not the noise/problem when you have the chance I would remove the valve covers and clean the oil tubes and replace the plastic pieces that hold the tubes to the camshaft bearings. Very brittle after all these years and heat cycles. May as well check the chain wear and chain guides while you are in there. Mark