When it is in park, the noise isn't there. When I shift to drive or reverse the noise starts. Its almost as if when I put it in gear the pulley has less tension on it and sounds like its flapping a little. If I rev it at all the noise goes away.

After driving it for a while yesterday, the noise wasn't as noticable when I pulled into the garage. I'm hoping maybe the new belt just needs to break in.

If it keeps happening, I'll take the belt off and spin it by hand to see how it feels as suggested.