A worn-out U-joint "needle" bearing can make this kind of noise. If it is the front U-joint it might make you think the sound is coming from the engine. Might be at least another 5000 miles or so until symptoms of vibration appear. Quite difficult to diagnose in the early stages, there might not be much play to feel for in the U-joint. Replaced a couple of U-joint crosses/bearings. First one you do is a bit difficult, then it becomes rewarding fun. Have somone advise you if you haven´t done it. Penetrating lubricant, a pretty big vise, and some regular sockets to press out/counterhold the bearing "cups" with is all you need.

This is my bet, with the modest degree of info available. Wow, Jarrod, if we could send in recorded sounds from our "cases"!