Subject: Why do squash balls sometimes make a weird high pitched noise when hit?

I've been playing squash a fair bit recently.  We play with a yellow dotted 
ball if that makes any difference.  It's strange, every now and then you hear 
this bizarre high pitched sci-fi noise that seems to come from nowhere.  In 
time we figured that it must be from the ball, though I guess since it travels 
so fast it makes sense that by the time the sound reaches us the ball is half 
way across the court.  Can anyone explain what causes the sound?

Re: Why do squash balls sometimes make a weird high pitched noise when hit?