Why won't the cows stop mooing..why are they mooing?

There's a pasture beside my house and across the street from my house. Well this huge herford(sp?) bull keeps mooing this moo( that sounds like a cellphone vibration,a steady beat, ) to the angus cows across the street. Literally for hours straight! It's annoying and keeping me awake. Is it perhaps a cow in heat? If so when will she come out of heat? Thanks!