I've have been hearing this noise since day one as well (2013 Premium Sedan - 5 speed).
When inside the car with the windows up it sounds more like a hissing noise. Almost like an air leak or something.
Outside of the car it sounds more like a metallic squealing noise.
Sounds worse (louder) if there is something for the sound to reflect off of.. like the side of a building or something.
Only happens at light, partial throttle.
Doesn't seem to be linked to engine speed (RPM) as it will happen at any RPM as long as I am lightly pressing the gas pedal.
I notice it the most when I'm driving down the highway at around 50 mph and need to give the car a bit of gas to maintain speed. It chirps.
I haven't been able to duplicate the noise while in neutral. Can only duplicate it when in gear and driving down the road.
I'm not convinced that it's the air intake.
If that was the case I think I would be able to duplicate it in neutral.
But I could be wrong.
I'm wondering if it's a whiny fuel pump or something.
There is a TSB out for a similiar sounding issue in the BRZ (09-54-12R:High-Pitched Chirp, Squeak or Cricket Sound from the Engine).
Wonder if it could be related (similar part).
The CVT models have an extra noise insulator attached to the hood lid
Perhaps that would explain why CVT owners haven't noticed the noise?
Or maybe the CVT's inherent whine is enough to mask this noise?
My car is just about due for its first oil change. Going to bring it up with the dealer then. However, based on past experience I'm pretty sure the dealer will just tell me its normal or that they cannot duplicate it.
All in all it's not a terrible noise and I've already gotten used to ignoring it.
But I would prefer to not have to hear the chirps and squeals for the next 100,000 miles.