Am I gonna need a new tranny?

[cameronmcclure]

So recently my truck has been making some noises that worry me. When Im running down the highway between 57 and 65mph and I let off the gas and let the truck coast, it growls. Between 57 and 65 is the only speed it does it and only when I let off of the gas. It doesn't squeal or sound like metal on metal. It doesnt effect the truck in any way it just worries me. Any insight would be greatly appreciated. Oh ya when I checked the tranny fluid there was some reddish looking gunk on the end of the dipstick. I read on here or the Nation somewhere where somebody else was having the same gunk. Just looking for some advice. Thanks.