When I turn on my ford Explore it makes a roaring sound. What could it be?

Does it get quieter after the engine runs a few minutes? Are you listening to it with the hood open now rather than from the drivers seat? It's possible it is just the 'clutched' radiator fan that makes a roaring sound when the engine is first started, like they all do, and you are just more aware of it now while you are checking stuff out! (Possible) Best thing, i think, is before doing anything else, take it back and have the installer check it out.

AGREE WITH PUSHROD

hi you might want to have who ever installed this alternator to recheck it maybe a defective part or something not installed right especially if noise was not there before