Recently noticed a hissing noise coming from the passenger front-side of the engine compartment (alternator area). The noise only appears when the car is warmed up and revved to over 2k rpm. At that point the hissing starts and does not stop even when idling. As soon as the engine is turned off (even when warm) and re-started the noise is gone (until 2k rpms). Any ideas?

 

Vacuum leak?

 

Maybe a snake?

 

probably a dry/cracked PCV hose - check it first. Also check the air intake tubing, I have seen those split and only notice it when removed from the car. Some hissing is normal from the fuel system.