I'd guess is was suspended sand rubbing on the tubes which is common at high flows. I head it on the Main Salmon last week but not the clearer Middle Fork
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I've had the same occur in canoes... I was once told that the noise is created because of the velocity profile of the river being faster at the surface and slower at the bottom of the river due to frictional forces.Heard it as well on ruby horsethief. Drove me nuts trying to find the hole till I figured out what it was.
The kayakers on our trip showed me a cool trick, you put the paddle blade in the river and put your ear to the shaft or other blade and you can hear the same sound.
What you heard is likely a combination of several things already mentioned. Saltation is a definite possibility, though that term isn't always used when the fluid medium is water.Or, you could be hearing the sound of saltation?
Saltation (geology) - Wikipedia, the free encyclopedia