[V8] 5000 transmissions

[Chris Brown]

 I know this os off topic but i also have a 1987 5000 quattro. 

 Tonight i took it out on the interstate and went about 100 mph for about 1 minute  just to get her rpms up and it ran fine. However when i got off the interstate and as i has heading thru the back streets to my house, every time i downshifted from 4 to 3 i got a grind, in the tranny. I had the clutch all the way in and still got it. It was quite loud. All the rest of the gears are there including 3rd but i got a *new* sound and a very scary one at that.  

  Does anyone know what i should do? Im suppose to be selling her this weekend and that noise was NEVER there.