2004 Jeep Grand Cherokee LE. This has only happened twice: driving between 35-50 mph when suddenly a loud sort of fluttering sound accompanied by a loud whine. The whine evoked the picture of a straining engine, as if something were dragging inside it. This lasts only until the car stops, and is gone once the engine is restarted.
Happened once a year ago, mechanic couldn't reproduce it. Finally happened again last week, but again disappeared after restarting the engine.
It's such a violent sound I worry something very damaging is occurring, but there's no consistency to it.
Note: The fluttering sound can also be heard some when accelerating uphill, but the straining noise has only happened those two times.