Female Cuckoos have a 'bubbling' call, not often heard, and very different from the male's song. Check out http://www.xeno-canto.org/181351
Yes, it was the distinctive bubbling or chuckling call of a female, from the reeds below the tree the male was sitting in.
Heard by me once, 04.30, right outside bedroom window. I did not have a clue what it was at the time, sounds "tropical"......
Many thanks. Amazing!