I'm getting feedback in my (white) common wire. I have a three wire that feeds a bathroom. The three wire originates at a switch plate which is fed from the breaker and another hall light is fed from the same home run. The three wire was existing into the bathroom.

At the switch plate, the black is hot and the red is switched. In the bathroom, there is a GFCI receptacle and the black and white wire are the line and the red wire runs to feed another hallway light. The load side of the GFCI services the bathroom, all protected. One vanity light and a fan light over the tub/shower. The rough wiring is done and the fan light installed on one switch and the vanity light on another.

All seems to be working fine, however, the vanity light isn't installed yet. The light switch is off and I cut the vanity switch wire and the GFCI tripped. That struck me as odd, so, I reset the GFCI and tried to trip it again with the vanity wire black to white with the switch in the off position and it didn't trip. The I touched black to ground, and it didn't trip. Then I touched the white to ground and it tripped.

Any idea what may be causing the white to ground tripping the GFCI? Where should I look. I checked and rechecked all of the wiring and it appears good...