Voices in my head...

…Or, more accurately, Chief’s helmet.

Cortana and the Gravemind both communicate to Chief throughout Halo 3, apparently stunning him and distorting his vision.

Then in Halo 4, the Didact is somehow able to talk to Chief without Cortana picking up any readings.

THEN during the last push of the level Midnight, after losing Cortana’s chip to the Mantle’s Approach Chief can still here her rampant whispers.

Any explanation?

The distortions in the beginning of H3 might have been his guilt on leaving Cortana behind, when thwe Gravemind speaks to him he uses the Flood Combat forms to talk.

He could hear the Didact as they are communicating telepathecally due to the evolutionary boost he recieved from the Librarian. After Cortana is destroyed she can still hear her faintly as she is still apart of Mantle’s Approach