Mortgage rates hit new lows again this week, driven by growing concerns about the health of the U.S. economy that have also fueled speculation that the Federal Reserve will soon embark on a third round of "quantitative easing," or QE3, to encourage borrowing and fuel growth.Freddie Mac's weekly surveys of lenders show rates on 30-year fixed-rate prime mortgages available to borrowers with good credit have been below 4 percent during all but one week of 2012. Rates on 15-year fixed-rate mortgages, popular with homeowners who are refinancing, have been below 3 percent for the last eight weeks.Although low rates have spurred another refinancing boom, many would-be homebuyers fear the possibility of another economic downturn, or are unable to obtain a mortgage, limiting the beneficia...