Acceleration shifting issue

[Coach]

The transmission in my truck is a 700r4. About 2 months ago, I came to a complete stop and when I began to accelerate and it shifted to 2nd gear, there was a loud noise that appeared to come from right under the floor where the shifter is. I thought it may have been the TV cable ratching loose, but wasn't. Nothing else after that time. Then in the last week, it has happened 3 times intermittenly. The best way to describe the sound is a loud bang or tension snapping sound. No problems at idle or when shifting from park to reverse or drive. It only occurs when accelerating from a complete stop or when you almost come to a stop at a red light, it changes to green and then you accelerate. Any suggestions??? Possible engine or transmission mounts???