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Infiniti 2003 G35 75k miles

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?

These kinds of things are very hard to diagnose standing right there, let alone on the internet. Is there anything that’s not working right?

Hissing coming from the engine compartment would often make a mechanic think of a possible vacuum leak. Vacuum leaks can behave strangely like this, coming and going for no apparent reason. That’s a good place to start. Your shop may have a device to help pinpoint where the noise is coming from.