Oil, probably valve gasket. If you buy the gasket valve set ($60 or less at Amazon and RockAuto) you will want to have your Valves adjusted while they have everything apart. Also good time to replace Spark Plugs.

Hissing might simply be the nut underneath the EGR valve this is known to shake loose often, and happened to me. It is just below the EGR valve around a round metal tube, and pushes back up under the EGR valve area and screws back in place.