According to Freddie Mac -Mortgage interest remained near annual lows last week, with the 30-year fixed rate averaging 4.51 percent compared to the year's low of 4.49 percent reached three weeks ago. The 15-year fixed rate, meanwhile, stood fast at 3.69 percent after hitting its 2011 low of 3.47 percent two weeks ago. Trackers say expiration of the central bank's $600 billion bond purchasing program -- which pushed down Treasury yields and, subsequently, mortgage rates -- could mean higher interest later this summer.