2002 Ford Focus blower

Air Conditioning problem
2002 Ford Focus Engine Size unknown Wheel Drive Type unknown Automatic 134000 miles

Problem started many months ago, it was just sporadic then, but worsened lately: whenever I turn on heat, A/C or defrost, at a 3-4 power setting, it starts making a loud almost screeching sound and then starts blowing air out in bursts. It will do this until I turn it down to level 1 or off. What may be the cause?