It appears to be the F above middle C on the upper keyboard. It was set on Chimes the first time, and I changed the setting to the Tab (Organ) button and it still sounded, but with the organ sound. I have not had a chance to do much playing around with it as the problem occurred during our church service both times. I practiced for approx. 45 minutes on Saturday and the problem didn't occur then. So far, ony during church service.
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lynnsue