Amid ongoing efforts by the Federal Reserve to lift billions of dollars in agency mortgage-related assets from banks' balance sheets and the Administration's intent to push interest rates down, mortgage rates across the US continue to hover near historic lows. Fixed mortgage rates slipped in the week ending May 21, according to Freddie Mac's (FRE) weekly survey. Thirty-year fixed mortgage rates averaged 4.82% with an average 0.7 point, down from 4.86% last week, while 15-year fixed rates averaged 4.5% with with an average 0.7%, down from 4.52%. Adjustable-rate mortgages (ARMs) were mixed in the week ending May 21, with 5-year Treasury-indexed hybrid ARMs averaging 4.79% with an average 0.6 point, down from 4.82% last week. At the same time, 1-year ARMs averaged 4.82% with an average 0.6 point, up from last week's 4.71%. “Long-term fixed-rate mortgage rates have remained below 5% for the past 10 weeks as the US Treasury and Federal Reserve act to keep interest rates low through security purchases,” says Freddie's chief economist Frank Nothaft in a media statement today. The Fed through mid-May bought $740 billion in MBS, according to Nothaft, pushing its balance sheet to $2.12trn in the week ending May 13, up $1.25trn from the year-ago week ending May 14, 2008. A separate rate survey conducted by found conflicting results, however, as the 30-year fixed mortgage rates it studied averaged 5.24% the same week, up 3bps from the previous week. More in line with Freddie's results were 15-year fixed rates, which averaged 4.74%, 2bps down from last week, according to Bankrate's survey. The monthly payment on 30-year fixed mortgages, which grew in popularity for home purchases after borrowers found refinancing out of increasingly expensive ARMs increasingly difficult, averaged $910.11, Bankrate found. This average sits $3 above last week's average but still historically low, indicating high levels of affordability. But even if prospective buyers jump at the opportunity, stricter underwriting standards among lenders often make for a drawn-out closing process, according to Bankrate's mortgage report author Holden Lewis. "While mortgage rates remain steady, wait times keep stretching longer -- specifically, the time that elapses between applying for a mortgage and closing on it," Lewis says. "Only an optimist would count on getting a loan funded within 30 days of application." Write to Diana Golobay. Disclosure: The author held no relevant investment positions when this story was published. Indirect holdings may exist via mutual fund investments.