2003 Ford Explorer heater clicking

for a while when i would switch from A/C to heat there would be a clicking sound for a few seconds before the what i think is some sort of "flapper valve" would finally thud. It got worse and worse, i would have to keep switching from A/C to heat to get it to engage. Now it just doesnt engage at all... It is stuck on A/C. what could this be? What has to be done to fix it?
Thx,
Paul