hissing noise when I accelerate
 

Today while i was driving, all of a sudden, i realized that my car was making almost a hissing/whistle noise, this sound is only there a little bit on 3rd gear and gets louder on fourth gear. What do you guys think it is. my car is standard. the clutch is oem and hasnt been changed, but the gear doesnt slip at all. stock intake. pretty much, everything under the hood is stock. It doesnt have power steering, so it cant be something related to that. anyone else have this happen to them?

Check for loose or leaking intake components.

Is it 100% stock? Like the air box hasn't been removed from the fender-well?

Is the noise dependant on rpm or throttle input?

well, recently i had my timing belt and water pump changed, but I drove it for like a week, there was no issues, so i'm thinking it has nothing to do with that.

yeah the noise comes when I accelerate, possibly after 2500rpm,
but its only there or I can only hear it in 3rd, mostly on 4th gear.