Strange Metallic Whirring Noise

Almost sounds like a couple keys jangling together. If there is nothing under the dash rattling around, and the pipes aren't banging around against the hangers or something then I would spray a little water on the belts and pulleys and such. If that doesn't quiet it down then it would be time for a stethoscope.

Also, there is a short period where it sounds like there could be some piston slap going on, but that isn't the noise your talking about.

Alternators have been known to make those strange erratic noises also.

I had a water pump damaged in a car/deer altercation with just enough damage to cause it to squeak for 40,000 miles before I finally tired of the noise and changed it out.

I know you seem to think the sound originates from the rear, but sound is a wave and the fan can blow it around, other sounds can interfere, and an engine compartment is like a tin can so it will echo all over the place.