NEW YORK (CNNMoney.com) -- Mortgage rates fell for the eighth time in nine weeks, according to a survey released Thursday.

The 30-year fixed-rate mortgage (FRM) averaged 6.40 percent for the week ending Sept. 21, down from 6.43 percent, according to Freddie Mac's (Charts) Primary Mortgage Market Survey. A year ago, the 30-year FRM averaged 5.80 percent.

The 15-year FRM averaged 6.06 percent this week, down from 6.11 percent last week. A year ago, it averaged 5.37 percent.

Five-year adjustable-rate mortgages (ARMs) came in at 6.08 percent this week, down from 6.10 percent last week. A year ago, they averaged 5.31 percent.

One-year ARMs averaged 5.54 percent, down from 5.60 percent last week. A year ago, the one-year ARM averaged 4.48 percent.

"A slowing housing market and signs that inflation is leveling off have helped to lower mortgage rates lately and keep them more affordable," said Frank Nothaft, Freddie Mac vice president and chief economist, in a statement. "For example, housing starts dropped to a three-year low in August and the Producer Price Index [PPI] fell below market expectations.

"Going forward, the economy is expected to expand at a somewhat slower rate than it did in the first half of the year. This should continue to keep inflation in check, and therefore, mortgage rates low."

Freddie Mac competes on the secondary market with Citigroup (Charts), Countrywide Financial (Charts) and Fannie Mae (Charts).

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