Depending on how much free time you have and how well you can program an Arduino, it's possible. I built one a few years ago using an "air popper" kit I bought off of thehauntstore.com, powered it with a 12v battery, then used a relay hooked up to an Arduino Uno with an ultrasonic distance sensor as the trigger. The Arduino detects a change in distance when someone walks in the path of the sensor, and triggers the relay to close the 12v circuit to the air popper for just under 1 second. Assuming you have a decent amount of air pressure, your guests will both feel a blast of air as well as that pneumatic hiss you want. If the sound isn't loud enough though, you could also have the Arduino trigger a MP3 or WAV trigger board as well. Good luck!