I would suspect a worn serpentine belt. Use a long handled screwdriver as a stethescope to maybe isolate the noise. Then I would remove the belt and start engine to see if noise is eliminated. Inspect belt grooves carefully and also look at idler pulley and other drive pulleys for any obvious wear. Is your belt original?

1995 Mercury Mystique LS OBD1 2.5 L Duratec ATX- No modifications 70,000 Miles w/ O.E. Water Pump or Replaced: VSS, Lower speedo-cable(2X) Use: Mobil 1 5w30 and Techron Fuel Treatment.;) and Mobil 1 ATF Change every 10,000 miles