DH is right. Steve Chappell has an estrus screaming cow call that he makes by using a mouth diaphram. I heard him talk about it in an elk seminar last year.
There is also awesome footage of a cow making the estrus scream on the new Extreme Bulls 5 video. It has incredible audio so you can learn what to sound like from a real cow. Chappell also has an instructional CD out that teaches how he makes the call with a diaphram.