While I don't have an answer, my washing machine repairman yesterday informed me of a cause (for over $150!!). My machine is fine, but the trees overhanging the clothes line drop a gum / sap / 'stuff' on the clothes which is not noticed UNTIL the next time it gets washed. It seems a laboratory confirmed this. Apparently, the washing detergent reacts with the gum to cause the stain. I still haven't found a way to remove the stains, and I welcome anyone's advice on how to achieve this.
D'Arcy Watson, August 2011