Does it sound like it's coming from the engine compartment or do you "feel" it more than just hear it, through the gas pedal, shifter handle, etc? If it doesnt do it in park....than it's probably not the engine. ( could be mains, but i doubt it...for they tend to knock)
When my rear pinion bearing went out....it was only heard when there was a load on it....as in accelerating or pulling a hill. Yours could be a wheel bearing, though. How about when you apply the brakes? Does the sound change or stop?