I recently replaced my valve cover gasket. I am now getting a slight hissing from either the cylinder 5 or 6 fuel injector (I think) along with a very subtle thumping noise from the engine. Is it possible that a bad injector is leading to a misfire?

 

Sounds like a vacuum leak? Spray some carb cleaner around the area making the noise listening for a change in RPM, while its idling.