That my friend is the start of a headgasket issue. I had one that would squeak at startup and proceed to do so for about 20-30 seconds then go away, it ran fine and raced well plus it would be ok when warm. But the problem got worse and it eventually had to be replaced. It did it again and the head had to be surfaced, then it was all ok. That was an open motor though.
You could try to retorque the head but I doubt it will help.