2004 Dodge Durango Knock

Noises problem
2004 Dodge Durango V8 All Wheel Drive Automatic

A knock has developed in the car when it is ideling it is fine when you go to accelerate it knocks betwen 1st and 2nd gear and after that it is fine no noise.

This is my son's vehicle and they do not have much money, the dealer said they had to have the car a for almost a week to see what was wrong and will cost the kids an arm and a leg. I was hoping you could give me some good news and some idea as to what it might be, they said something about transfer?
Awaiting your answer thanks.