"Keith Hughes" wrote in message
...
I've been asking myself the same question now for years :-) We're in a
slip on Harbor Island, and hear the same thing. Guess I have an answer
now.

Keith Hughes
And I'm on the Rock River in Northern Illinois. Have always heard the same
noise when anchored and assumed here that it was rocks rolling along the
bottom or carp feeding on the algae (or whatever) that builds up on the
bottom of the boat.
Tom.