2004 Ford Freestar Transmission rebuild?

I have a 2004 Ford Freestar with automatic transmission. On my way back from a trip this weekend as I accerated up hills I heard a rattling/racheting sound from underneath the car. I drove it home to my mechanic who says it is a tranmission problem. He recommened a shop to look at it. Does it sound like it would be a total rebuild? The car shifts and makes no noise if I don't accelerate hard or quickly.

Thanks for your advice