MYSTERY THAPPING RATTLE SOUND

Disconnect the drive belt and see if the noise goes away. Rack and pinion only should make noise while turning the steering wheel. Is it a heat shield or loose exhaust? It's not a lifter tapping either? Stethoscope can help greatly. Also check by the flywheel and starter to see if any obvious noises.

Bad idler wheel/tensioner?

Few updates based on comments: I removed the serpentine belt and ran the vehicle. There was no noise with the belt off. I rotated all the pulleys and they felt smooth. I put the belt back on and the noise came back. I used a stethoscope, but could not locate the source. The sound travels through the AC and power steering pump and the rest only sound like normal movement. The sound doesn't change at all when I move the steering wheel, so I think you're right that it isn't the rack and pinion. The heat shield is tight, but I didn't consider moving the exhaust, I'll give that a try. As far as a bad idler wheel/tensioner, the stethoscope didn't indicate any noise from there. Do they ever go bad in any way except failed bearings?

The Bearings do get louder when they do wear. If there is a worn or bad spot in the bearings of the idler or tensioner can cause them as well. When checking sound with a stethoscope you got to listen closely to notice any sound changes as well. Different RPM can change noises as well.