Average interest rates on long-term U.S. mortgage loans fell dramatically last week, making September the most volatile month since March, according to Freddie Mac. The average rate on a 30-year fixed-rate mortgage dropped to 3.64% with 0.6 point for the week ended September 26, down 9 basis points from the previous week and down much more from 4.72% the year before. As investor moods have been yanked around by global markets, September rates have had big swings up and down with weekly rate movement averaging 11 basis points.