2003 Ford Windstar Heater not working right

I have a Ford mini van and the heater has just started acting up.

The first thing I noticed is when I would slide the bar from cold to hot I would hear a clanking noice. But the heater still worked.

Now the only thing that works on the heater is the back heater vent that is by the left rear door. The front heater/defroster only blows out cold air. And when I put the rear control on rear control nothing comes out of the top vents.

I have tried all positons on the front heater control and the fan works but only cold air comes out.

When I use the rear control the only thing that works is the vent by the rear left door.

Does anyone know what's wrong with this heater?

:) Cyndi