UPDATE to Harley Making Noise
 

I've now noticed when It makes the flutter sound. Its when the automatic transmission is switching gears. Mainly from around 50mph to 60mph. Some times a lower gear though. I dont feel much of a jerk of the truck. Maybe a hint of lag.

Some would call it a grinding sound maybe. Sounds like its coming from more the middle to right back end. Any ideas fellas?

ps- im watching the rpms at around 2000 and the noise occurs while there dropping to about 1500 while accelerating.

Thought I would let the readers know... Ford says I need my tranny flushed. The fluid is broke down. And after reading on here I see that is typically the problem. Im getting that done monday... I will let you know.

Got the Fluid changed per the specs. Using the decoder I got off of this site to find out which tranny I had.

That fixed it!!!

How many miles did you have on the tranny fluid? I changed mine when I bought my truck at 22K and then again at about 40K. I talked to a tranny guy and he said it is better to change the fluid more often than to use synthetic fluid. Synthetic is better, but people who use synthetic fluid tend to go longer length of time between changes.

Get yourself a Factory Tech valve body, and a 4X4 transpan that has a drain plug. It will make future changes a lot easier and make your trans shift better. Glad your problem was minor.