if the tune is off enough it will cause a bad idle and surging because the o2 feedback is trying to correct a terribly lean condition.

the hissing is probably the air sucking in/through the intake and filter. reconnect the vacuum lines the way they were and follow this:

the bad idle/surge can also be because of a vacuum leak. to check for this pull your intake tube off the throttle body. take your hand and cover the throttle body completely. it will try and suck your palm in. if there is a vacuum leak your idle may drop, but it won't stall out. if you don't have a vacuum leak your idle will drop and the car will stall.

try that and listen to see if you still hear the hissing.

report back on your results.