[~The Jester~]

Could be dry bearings on the alt. Here's what I'd do. Have someone else start the car while you listen for the sound under the hood. Have them keep starting it while you zero in on the noise.

Bad alt bearings should "fluctuate" in sound though, different RPM usually makes different sounds. Also check that the alt bolt didn't come loose, and maybe the alt fan itself is hitting the casing. Pull the inspection cover, and double check your converter bolts, make sure they're all still tight too.