WASHINGTON – Mortgage rates fell to the lowest level in decades for the 10th time in 11 weeks, as investors worried about the economy.

The average rate for a 30-year fixed loan was 4.32 percent last week, down from 4.36 percent a week earlier, mortgage buyer Freddie Mac said Thursday. That’s the lowest since Freddie Mac began tracking rates in 1971.

The average rate on 15-year fixed loan dropped to 3.83 percent from 3.86 percent the previous week. That’s the lowest on records starting in 1991.

Rates have been falling since spring as investors have shifted money into safer Treasury bonds. That has lowered their yields, which mortgage rates tend to track.

The low rates have fueled a wave of refinancing by borrowers. Refinancing is at its highest level since May 2009 and makes up almost 83 percent of all new loans, its highest share since January 2009.

People seeking lower rates helped boost mortgage applications by 2.7 percent two weeks ago, the Mortgage Bankers Association announced Wednesday.

However, the low rates haven’t been enough to lift the struggling housing market. Home sales are at the lowest level in more than a decade. Potential buyers are holding off purchases, worried about jobs and the economy. Some are having trouble meeting tighter lending standards.

To calculate the national average, Freddie Mac collects mortgage rates Monday through Wednesday of each week from lenders around the country. Rates often fluctuate significantly, even within a given day.

Average rates on five-year adjustable-rate mortgages fell to 3.54 percent from 3.56 percent the week before. Rates on one-year adjustable-rate mortgages fell to an average rate of 3.5 percent from 3.52 percent.

The rates do not include add-on fees known as points. One point is equal to 1 percent of the total loan amount. The nationwide fee for loans in Freddie Mac’s survey averaged 0.7 a point for 30-year and 1-year mortgages. They averaged 0.6 of a point for 15-year and 5-year mortgages.