Cut your fan belt?
This is supposed to happen, your fan runs off your serpentine belt system and will always be spinning while your engine is on. This is the correct function unless your fan clutch is seized, which will lead to violent vibrations at high RPM and eventually turn your fan into a grenade. Ask me how I know.
You may notice its louder at start-up and quiets down after a few seconds. If it doesn't quite down you may have a seized fan clutch.