I have the exact same truck. Same year 4 by4 4.0 I hear a hissing noise as well and then it would shut off and then the defrosters would go on when I have it on regular air conditioning or vent. Then when I get off the gas then it comes out events how it normally should.

I have a mercury mountaineer same thing my ac works but when I get on freeways and hit like 70mph it stops blowing threw the vents and goes to dash vents any suggestions

Do these have a cabin air filter my mother in laws has a weird smell coming out but I can't find a cabin filter or any video on that specific thing

In my truck air is coming from outside even ac is not working