Is this a random noise or does it have a rythem to it?
A bad needle bearing in the U-joint will make a snapping sound when they start binding and breaking into small pieces. Jack the truck up and put the trans in neutral and see if there is any play in the u-joints.
Can you feel it in the shift levers?
Could it be a motor mount coming off of it's seat and then klunking back in place?
Grabbing straws here...
Mike B
