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I have a 99 f250 SD 4x4 with the 5.9L engine. just recently when im driving a grinding sound starts most of the time it happens when i slow down and its coming from the transmission i think. It stops if i stip completely or if i slow down and put in 4 wheel drive and take it out again. Anyone have an idea what it might be, thanks.
That looks like another typo or aftermarket conversion. In '99 the three engines that were available were the 5.4L V8, the 6.8L V10, and the 7.3L diesel. If you don't have one of these your truck has been modified.
Still, not as important as what transmission are you using?
Also, you said the sound stops if you "slow down and put in 4 wheel drive and take it out again."
Lock your front hubs (don't put it in 4 wheel drive, just lock the front hubs) and see if the noise goes away. I think maybe you have a front hub that went bad, not a transmission problem.