I have the 4.6L and I'm noticing this weird high pitched moan/humming. It only happens if I'm on the highway or a straight stretch. When I let off the gas it does it for maybe 3 seconds and then if I hit the gas again it happens. It sounds like it's coming from the back of the truck. I was thinking maybe the factory exhaust or the rear end. Any ideas? It's driving me crazy!

 

Sure sounds like the ring and pinion is going bad on you. I had 67 Stang that started doing that, it had the exact same symptoms. Sounds like good excuse to swap to 4:10's I'm sure someone will come along any minute and prove me wrong though. Happens all the time,,,,98