Here goes for a description:
Truck has 5k miles,no previous tranny/engine issues.

While driving about 35mph in top gear,easy pressure on accelerator to barely increase speed..strangest sound happened 3 times...3rd was with me trying to replicate. I cant get it to repeat it.

Sounds almost like a backfire noise. Almost. If any of you have bounced off a rev-limiter that actually shuts fuel off because of excessive boost...that kind of sound. No feel of shutter. Engine rpm's remain constant.
Seems to happen when tranny is in top gear and its at the threshold of downshifting. Very quick only lasting a second before you instinctively let off gas.

Anyone else experience this? Will call dealer in morning, but if I cant it to repeat it...

thinking back, I realize it probably wasnt in 6th gear, but 5th maybe. That really doesn't matter , I guess. Still, the highest gear possible for abt 35.

 

I tried to repeat the sound and couldn't get it, so I drive "normal" Bang! It does it again around 40MPH or so.

It really sounds like it i coming from the flywheel area. ODD!!!