I just need an idea of what the heck is wrong with this thing...

I have one of the big Heil ac/heater units that sits outside..

it has been working ok.. no major complaints.. then this morning I noticed that the house was getting warmer..

So I checked the floor vents.. no cold air.. so I checked the unit outside .. it was running and blowing alot of air out the top (where the big fan is)..

So I went under the house and checked the duct (the flexible kind).. no rips or tears that i could find..

Then I took a knife and cut into one of the ducts (the one that blows into the house).. there was a little cool air.. but not what you would expect to be blowing through the duct..
(I patched that spot btw)..

When I shut the unit off there was a "hissing" sound comming from the unit.. no idea if that's normal or not..

So I am guessing it has something to do with the fan that blows into the house.. not sure cause i know basically nothing about it...

The unit was here when we moved in.. and from what I was told it's a "commercial" unit.. we haven't had it serviced yet.. but that was on the "to do" list...

Any ideas?