I also have the humming noise in my 94. The new radio did not stop the noise from a new cd player. I only have it once in a while now.
The humming is apparently being caused by a bad ground that has something to do w/ the a faulty fuel pump. Ford did issue a recall on the fuel pump.
This problem happened only in 92-94 mustangs regardless of model.
Easy fix: go to Radio Shack and pick up a $15 noise supressor or have it installed by pro.
Good Luck
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Green '94 GT conv. -- K&N filter charger, FMS 9mm wires, Accel hipo cap/rotor, 180* thermo, 14* timing, PRO 5.0 w/ stock handle, off road H-pipe in front of 2 chamber flow's, Steeda subframe connectors w/ crossbrace, stock, stock, stock, yeah and that's stock too.
My 'Stang