When I remove the oil fill cap on the valve cover while the engine is running I can hear it "puttering" and feel the air pressure coming out of the oil fill hole. I was told this could be a sign of a leaking valve, but is this true of overhead cam engines as well? Should I worry about this? The car runs fine and doesn't skip or seem sluggish. If it helps this is a 94 Altima GXE that has 105k miles on it, auto tranny.