Hens turning into roosters?

I know it sounds crazy, but one of our hens has started acting like a rooster.  Pouncing on top of the other hens, fluffing herself up in front of them, chasing them around, and then the other morning while we were all outside enjoying the sunshine, my oldest son said, "shh, everyone listen!"  We all stopped what we were doing and listened, and what we heard was a very strange sound, like a gurgling, or a small engine trying to get started noise.  Then, after more listening we realized it was a crowing sound coming from the chicken coop.  Let me back up, and add, we have had chickens for about five years now, adding new chicks every spring to keep the flock producing eggs for our family.  Over the past five years, we have had a few roosters grow up from our baby chicks, so we know the sound of a young rooster just testing out their new lungs.  It is not a very appealing sound.  And ,since we live in the city in a neighborhood, they have had to make the move to the country.  However, this latest group of chicks, thanks to my husband's careful selecting, grew up as all hens.  And, after a bit of a slow start have all been laying eggs very successfully for us.  But, it seems one of them has decided it no longer wants to be a hen...I have read about this strange phenomenon once, but if anyone else has any info on this subject please let us know.  Not quite sure what to make of it.  The boys are totally excited, b/c as they said, "Mommy, since it crows softly and not early in the morning, we can keep it right?"  Guess we might be building an incubator soon?  Our very own spring baby chicks would be awesome!  But, again I do not know much about this whole sex change in chickens, so any info would be greatly appreciated!