Mortgage rates sagged this week as ongoing concerns about the European debt crisis had investors fleeing to the relative safety of mortgage-backed securities that fund most U.S. home loans.

Rates on 30-year fixed-rate mortgages averaged 4 percent with an average 0.7 point for the week ending Nov. 3, down from 4.1 percent last week, Freddie Mac said in releasing the results of its latestPrimary Mortgage Market Survey.

At this time a year ago, rates on 30-year fixed-rate mortgages averaged 4.24 percent before climbing to a 2011 high of 5.05 percent in February. Rates on the “plain vanilla” fixed-rate loan hit an all-time low in records dating to 1971 of 3.94 percent during the week ending Oct. 6.

Rates on 15-year fixed-rate mortgages averaged 3.31 percent with an average 0.7 point, down from 3.38 percent last week. The 15-year fixed-rate loan averaged 3.63 percent at this time last year before climbing to a 2011 high of 4.29 percent in February.

The the 15-year loan, a popular refinancing option, hit an all-time low, in records dating to 1991, of 3.26 percent during the week ending Oct. 6.

Rates on five-year Treasury-indexed hybrid adjustable-rate mortgage (ARM) loans averaged 2.96 percent with an average 0.6 point, down from 3.08 percent last week.

The five-year ARM averaged 3.39 percent a year ago before hitting a 2011 high of 3.92 percent in February. Rates on five-year ARMs tied an all-time low, in records dating to 2005, of 2.96 percent — last seen during the week ending Oct. 6.

For one-year Treasury-indexed ARM loans, rates averaged 2.88 percent with an average 0.6 point, down from an average 2.9 percent last week. At this time last year, the one-year ARM averaged 3.26 percent before climbing to a 2011 high of 3.4 percent in February.

Rates on one-year ARM loans hit a low, in records dating to 1984, of 2.81 percent during the week ending Sept. 15.

Looking back a week, a separate survey by the Mortgage Bankers Association showed a modest increase in demand for purchase mortgages.

The MBA’s Weekly Mortgage Applications Survey showed requests for purchase loans were up a seasonally adjusted 1.8 percent from the previous week, ending Oct. 28. Demand for purchase loans was down 2.1 percent from a year ago, and requests to refinance accounted for 77.1 percent of all mortgage applications.

In an Oct. 17 forecast, economists at Fannie Mae said they expect rates on 30-year fixed-rate mortgage loans to average 4 percent next year and 4.2 percent in 2013.