2002 Kia Sedona engine/drive noise on a 2002 Sedona

Okay this is an interesting issue I am having with my 2002 kia sadona mini van. Every time I first drive it after it has sat for a good long time (more then 5 hours) when I put it into gear, either D or R I hear a sound from the engine area until I fully accelerate. I can leave the car in park or neutral and rev the engine and don't hear it. Once the car warms up I never hear it at all. The only time I hear this sound (like a grrrr sound) is when the van is cold and between idle speed and say 5 miles an hour, because as I excellerate the sounds goes away. What might this be?