What would cause the A/C air to blow out from the firewall?

I was recharging my a/c, the compressor came on, I thought everything
was going fine, then I put my hand in front of the vent and the air was still
warm, it never changed. I thought maybe I just needed to finish filling it up
when the compressor went off and then I heard, what sounded like a loud
fan coming from the firewall. So the compressor is off and instead of air
blowing out the vents its blowing out at the firewall. I then checked the
pressure and all the refrigerant I had put in was gone. I tried again to
recharge it and same exact thing happened, but I couldn't find a leak, and I
don't know why there would be loud fan noise at the firewall