Got a 96 gmc nd i have a small hissing sound come from the engine while driving

It all depends on where the hissing comes from. But my understanding is that mass air flows do not make a hissing. They are a sensor so there are not any moving parts. And as for the transmission, Maybe there is a vacuum line you left off or something of that nature. I don't know your truck specifically but i know vacuum lines make a pretty good hiss and changing the transmission is a pretty large job and very easy to leave something like that somewhere in the engine because they are small.
hope this helps

Can you hear a hissing sound? by saying the engine dies when you apply the brake makes me think you may be loosing vacuum and the brake booster my be bad.This will make the engine stall.

Sounds like you have a vacuum leak, A dry rotted hose with cracks is usually the problem. Listen from where the sound is coming from and you will be able to pin point a small area of vacuum lines. Good Luck

Pressure test the cooling system, then fix what that shows.

Your coolant level is low, and you hear the coolant taking up the vacant areas....bubbling...due to low coolant.

replace the tps throtle possion sensor

Check fluid level when hot. If ok the flush cooling lines between tranny and radiator. Also check your modulator they are cheep from napa. Difficult for me to duplicate hissing sound on the computer but check your converter for how hot it is after twenty minutes. Last check the vacume line from manifold to modulator..sometimes a small hole can make that noise and effect shifting...hope this helps:)

LISTEN FOR HISSING UNDER DASH ON DRIVER SIDE. SOUNDS LIKE YOU HAVE A VACUUM LEAK IN BRAKE BOOSTER.

IDLE POSITION CENSOR IT IS ON THE OUTSIDE ONTHE SIDE OF YOUR TBI DISCONNECT THE WIRING IF THE IDLE SMOOTHS OUT IT IS YOUR PROBLEM IF YOU LEAVE IT UNPLUGED IT WILL RUN FINE BUT YOU WILL HAVE A CHECK ENGINE LIGHT

The noise maybe the p/s pump fluid return,GM had some problem with this but nothing bad. GM makes a resticter that goes in the return line of the pump, it will slow the fluid down going back in the pump stopping the hissing sound.