Mortgage rates tumbled this week, posting the largest weekly drop in the past year, mortgage buyer Freddie Mac said Thursday. Freddie Mac's latest Primary Mortgage Market Survey, released Thursday, showed the average rate on the benchmark 30-year fixed mortgage fell to 6.35% from last week's reading of 6.5%. The average rate on a 30-year loan was 6.2% a year ago. "Mortgage rates are headed in the right direction and homebuyers have noticed, as purchase applications reached the highest year-over-year grow ...