I have a 99 mustang v6 and I’ve had it for about 3 weeks and it has 122,000 miles on it, I changed the air filter fuel filter spark plugs wires and coil pack, I am having a hissing sound under the hood now seems to be coming from passenger side but I do not know where could it be the iac? Or the pcv valve connecting to the air intake, or could it be a exhaust gasket? Is there a vacuum line diagram, and my car makes a hiss when I start it and it revs, but revs to 4 k and back to 3 then 4 in neutral I do not know where to start or what to get any help would be appreciated