The 15-year is particularly popular with homeowners who want to refinance their old mortgages to a lower rate and pay off their loan more quickly.
Interest rates on 30-year loans, which are popular among first-time homebuyers, averaged 3.37%, a single tick above the record low of 3.36% set two weeks earlier.
Rates have inched down about 0.2 percentage points since the Federal Reserve announced plans in September to buy as much as $40 billion a month of mortgage-backed securities until the economic recovery started gaining momentum.
That may be happening already, according to Keith Gumbinger, of mortgage information company HSH.com.
"If the economy continues to show signs of improvement this fall, mortgage rates could firm a little more," he said. "For that to occur though, we'll need a lot more evidence that forward momentum is building."
Frank Nothaft, Freddie Mac's chief economist, said rates remained unchanged this week as "home construction builds up steam."