04 expedition 4.6L max a/c switches from vents to blowing out defrost area

When on max a/c and i accelerate quits blowing out vents
and it blows out defrost area.switch it to blowing only at
face i accelerate it does the same thing other than when i
let off the gas it starts blowing out vents again. Until i
accelerate . what could be the issue? Also the rear a/c has
a thumping noise every now and then (might do noise like
5 or 6 times then it stops) also the air thats coming out of
front is cool for a lil bit then goes a lil warmer. The rear  
was cold a few weeks ago now its warm. What could be
the problem? I did just replace the compressor and had it
vacuumed out also they filled up the freon.