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

Mortgage buyer Freddie Mac says the average rate for a 30-year fixed loan was 4.32 percent this week, down from 4.36 percent last week. 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 shift money into safer Treasury bonds. That has lowered their yields, which mortgage rates tend to track.