Primary Indicator: Unplug ALL devices on this circuit. Does the GFCI reset now?
Primary Indicator: Do the lights get brighter/dimmer when a heavy load (AC/Dryer) turns on?
Primary Indicator: Did this happen immediately after plugging something in or turning on a switch?
Primary Indicator: Are there multiple high-draw devices (Heater, Microwave, Hairdryer) on this circuit?
Primary Indicator: Is the face of the outlet warm or discolored (brown/black)?
Primary Indicator: Do 240V appliances (Stove/Dryer) work, but half the 120V lights are off?
Primary Indicator: Can you isolate the sound to a specific breaker?
Primary Indicator: Does it trip immediately when a specific device (Vacuum/Treadmill) starts?
Primary Indicator: Does the shock happen when touching the metal casing of the appliance?
Primary Indicator: Is this outlet switched (controlled by a wall switch)?