Either a pipe isn't connected tight or a leak in the compressor. If you're very certain it's the compressor then better have it exchanged. If not & don't trust the ol' place, have it diagnosed somewhere else. I went to this place and was happy with their work: http://forum.lowyat.net/index.php?showtopic=2152035.

If you can't fully twist the knob it might be caused by a sticking cable. You could DIY remove the glove box, and then wiggle or pull the cable atop the blower to find out where it's stuck.