I have a 1988 f-150 4x4 with a 5.0L which makes a kinda loud roaring noise when I left off the gas when going faster than 55 mph. The roar gets quieter as the speed decreases and eventually does away after you get down to around 50 mph or so. It does not do yhis at a lower speed only at speeds over 55 mph. I thought it might be the torque converter but it does not slip at all at any speeds. Any ideas what this could be?

 

It is possible that your thermostatic fan clutch may not be disengaging at high speed and is causing this noise.

When the engine is off--try turning the fan blades--there should be some resistance--but the blades should turn. If they dont--the fan clutch isnt releasing.