tranny driving me nuts

I have a 1997 ford explorer V6 aw/4w drive.For the past month about once a day when I am driving it, it screams bloody murder. It sounds like a loud motor running almost like a belt winding or a weed wacker. When it does this I put it into neutral and it immediately stops. If i hit the gas when it is doing this it becomes even louder and revs like i'm in neutral. It does it most of the time in reverse. Lately though I can be travelling down the highway and start to slow down to about 50mph and it does it. I drive about 60 miles a day and the tranny is right there it does not slip at all. Everytime I have someone look at it the damn truck wont make the noise. Everyone is telling me to wait till it gets worse, but i dont want to be stranded on the side of the road. Any ideas???

Does the tach show the rpms increasing in relation to this noise?

yes