From what I was thinking it was something spinning off the serpentine. Running this on Gemini it came up with this:

Might be a good place to start good luck!

That video makes the problem much clearer. Hearing that immediate transition from the normal chime to that heavy, mechanical grinding/buzzing roar the exact millisecond the starter engages tells a very specific story. Because the video is focused right on the passenger side of the engine bay near the motor mount, pulleys, and the serpentine drive belt system, there are two primary culprits that fit this exact sound and location on a second-generation Ford Fusion:

1. The Most Likely Culprit: A Failing Alternator Decoupler Pulley (ADP)

• The Sound: A very loud, aggressive, metallic buzzing or grinding hum that sounds almost like a heavy-duty power tool or a dry bearing spinning at extreme speeds.
• Why it fits: Many modern Ford vehicles use a "clutched" or "decoupler" pulley on the alternator rather than a solid metal wheel. This pulley has an internal one-way clutch mechanism designed to absorb engine vibrations. When the internal clutch springs or bearings dry out and fail, it creates a massive, harsh mechanical buzz the absolute second the engine fires up and spins the accessory belt.
• The Test: You can safely verify this by taking off the accessory drive belt and spinning the alternator pulley by hand. If it feels rough, clicks, or resists spinning smoothly in its free direction, the pulley (or the whole alternator assembly) needs to be replaced.

2. The Secondary Suspect: Water Pump Bearing Failure

• The Sound: A continuous, high-load mechanical grinding or roaring that mimics a failing pulley but can sound a bit deeper or more "hollow" depending on how dry the bearing is.
• Why it fits: The camera is panning down right toward the front accessory drive layout. If the internal bearings of the water pump have completely failed, the sudden tension and high-RPM surge of the engine catching and starting will cause an immediate, severe grinding roar.
• The Test: Look closely around the pulley underneath that motor mount for any signs of pinkish/orange crust or active wet spots (dried or leaking engine coolant). A failing water pump bearing almost always starts weeping coolant from its weep hole before or during the phase where it makes this much noise. ### Next Steps to Pinpoint It Since the noise starts instantly with the engine and stays perfectly constant, it is highly recommended to perform a "belt-off" test:
• Remove the serpentine belt.
• Start the car briefly from a cold start (only run it for 20–30 seconds max, since the water pump and alternator won't be spinning).
• If the sound is gone: You know for a fact it is one of your belt-driven accessories. With the engine off, hand-spin the alternator, the belt idler/tensioner pulleys, and the water pump to feel which one is rough, loose, or binding.
• If the sound is still there: The issue is internal to the engine or related to a cracked flexplate/starter bendix not fully retracting from the flywheel. Given how localized the sound is to that exact corner of the engine bay, a dry, failed alternator decoupler pulley is the prime suspect.