Rattle ceremony?

Went for a nice walk and lounged comfortably on a board seat that was wedged between some tree branches for a while.  As I was leaving, I heard some strange shaking/rattling noises.  When I got closer, I saw a man and a woman sitting in some lawn chairs, and each one was facing the stream and shaking a wooden(?) rattle. Also, there appeared to be some kind of white cord or rope which crossed over the water from where they sat, and it was tied or hooked around some round rocks in the ground on the other side of the stream. I kind of wanted to stop and stare, but thought maybe that would be too intrusive? The rattles might have had bumps, or were curved outward and inward several times, with some colors painted on the curves/bumps. I haven't found a similar picture of that sort of rattle on the internet. I am curious as to what they were doing...