It has to do with the hose you use without a doubt. I never had this until I bought Gardiner's micro bore. First day I experienced this was the first day using it, to this day, it still foam's all the time. I don't pretend to know why it does or why my previous hose's never did. Probably due to manufacture material probably. Thankfully, the good new's is it does not affect the end result, no matter how concerning it look's at the time of cleaning.