I was an AR for a U-12 game this past weekend, and only after the game did something click. When yellow was going for a header, white would yalp, you know a short yell. Yellow would then back away from the ball. Is this something an AR should raise his flag over, if the CR didn't call this foul?
Let the CR handle it, although you may want to have a word with him/her at an appropriate break or halftime.
Yelling or yalping is not a foul. It may be misconduct. Other than that, I agree with nsa's advice. This conduct would be good item for the pre-game if you happen to see this team again. I can't imagine the opposition backing off more than once.
It is a problem, typically among the younger players.. They scream when an opponent is about to beat them to the ball. Many times younger players feel as if the screamer thinks they are going to collide with them and since they lack the vision to see where the screamer is, they hesitate, back off or what not and the screamer may get the ball. I've seen it too often to count. It is misconduct, not in the spirit of the game, unsportsmanship, whatever you want to call it. The CR should deal with it. You should let the CR know about it at a break in the game when you can get his attention.