Question #1 where exactly do i find the (heater valve) on my truck.. on 2004 Ford Explorer

my truck had great heat a month later gone ..Now the blower still blows air ,however no heat and there is a qiet knocking sound .and if i dont keep the climate bottun on cold you will here this sound,right around half way to max heat.blower blow's fine .how do i check the heater valve...

Asked by for the 2004 Ford Explorer