Engine Whine

Can anyone tell me what would make a sound like a jet turbine winding up on my '01 4.0L Sport Trac? It started as I was leaving a stop light and increased in pitch as I accelerated. When I let off of the gas, the pitch stayed the same until I was almost stopped, then the noise stopped just like I'd flipped a switch. It didn't do it the rest of the day (about 85 miles.) This was the first time I'd heard it though my wife says it's been doing it ocassionally. It was loud with the windows up, and unbelievably loud with the windows down. If it was an aircraft, it would sound pretty cool, but.............. Any ideas?? H