G above middle C periodically playing loud and shrill

Out of the blue, the G above Middle C will everyone once in a while sound the same note at the loudest setting. I could be holding down the key and hear what I expect, a G note at the volume set. Within a few seconds of holding the note, another note will play much much louder. Sometimes that “overtone” will be one sustained note, sometimes a staccato like pattern, and other times just a one note. Always the same G and always much much louder. Disrupts playing and often very startling! Thanks for any help.