Weird 'turbo' sound?

[Leash]

The coupe's been making funny sounds for my bf (I haven't driven it since its been making the sound.) but when you semi floor it, right around 3k its makes a loud 'phisst' turbo sound when it switches to 2nd gear.

Its only happens in 1st and when he either floors it or semi floors it.

Its an auto '89 coupe, ka24e with aftermarket intake (installed about a year ago) and an s15 exhaust. And the car does not have turbo.

The alternator was also replaced about half a year ago as well.