If it rocks, it is bent. Cast aluminum valve covers cannot be straightened, they need to be replaced. Sheet metal valve covers can be straightened. A symptom of a bent or leaking valve cover is a pinching of the valve cover gasket. This means that the gasket is sealing one area and not sealing another area.
Some valve covers are hard to access, because they are covered with other engine parts.
Chronic valve cover leakage can sometimes be fixed by using two gaskets glued together instead of using just one. Valve Ports Valve ports are openings in the cylinder head. Intake ports let the fuel mixture into the cylinder head, and exhaust ports let the exhaust out. The valve's job is to open and close the valve ports. If the ports were always open, the fuel exploded in the combustion chamber would leave through the ports.