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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
of course when you introduce a load into the system the fuel pump noise will change = normal, now if you here noise through the radio speaers such as a buzz or whine, change the plug wires to a radio supression core style
Your turn signals shouldn't be drawing too much power. I would check the ohms from the ground side of the fuel pump wire to a good body ground. same with the power to the positive side of the battery.
take your underdrive pullies off and put the stock pullies back on and the minor voltage fluctations that the turn signal makes will be made much much smaller.