Just like Upallnight had mentioned, your grommet for the spark plug on the center part of the valve cover is perished. Unfortunately you need to remove the valve cover and replace the grommet; which would mean replacing all the center grommets and the valve cover gasket as well. Not a problematic job for DIY, or you can have a professional do it if have enough money in the kitty. Plenty of instructions have been published and you tube videos. Hope this helps, mate!