Go outside to your Mustang... leave the ignition key inside the house.
Pop the hood. Grab the fan blade and try to move it from side-to-side. Do the same thing for the A/C compressor, the power steering pump, and any other accessories where the belt meets the pulley. There should be no 'play' or 'give'.
Now, go and get you ignition key and start your Mustang... leave everything else 'as is' (with hood up and transmission in Park). Turn the steering wheel side-to-side, (check for noise) then increase the RPM's to about 1500 and turn wheel (Ck for noise). Then back at idle, turn the AC on (fan /low) ((Ck for noise)) and then increase to 1500ish RPM's (Ck for noise). Now, turn the AC fan on high, headlights ON (bright/high beam), fourway flashers (Hazard lights) and any other accessory to electrically stress the alternator (Ck for noise as rpm increase).
Drop the engine back to idle and go and get a clean white cloth or white paper towel. Pull out the trans dip-stick and wipe it clean and re-insert the dip-stick. What does the white towel look like? It should be pinkish to blood red. Now remove the dip-stick again and check the fluid level. It should be within the hash marks (idling level and in Park).
I am so sorry that this post is long, but noises are one of the hardest things to diagnose over a computer.
Good luck, Dave
P.S. (note) I ASSuME that within 8years you HAVE had your power steering fluid flushed and changed your transmission filter and had your transmission fluid flushed.