Home loan rates below 5 percent, their lowest level of the year, are fueling a refinancing boom.

"I've been flooded with phone calls," said Campbell mortgage broker Rob McCarthy of 101Loan.

A report from mortgage financing company Freddie Mac said average rates for 30-year, fixed-rate loans sank to 4.78 percent for the week ending Thursday. That was down from 4.84 percent the previous week, and just a smidgen above the record low of 4.71 percent set in December.

"Nobody ever thought they'd see a rate with a 4 in front of it again, but here we are," said Faramarz Moeen-Ziai of Bank of Commerce Mortgage, headquartered in San Ramon.

Just as they did five months ago, brokers say homeowners should jump at the low rates — including those who have seen their equity increase with rising home prices — noting the market is "volatile," changing frequently with investors' fears and hopes for the economy.

Concerns about serious debt problems in Greece, Spain and Portugal have driven investors out of stocks and into the relative safety of bonds in recent weeks, pushing bond yields down. Mortgage rates, which tend to mirror the direction of long-term bond yields, have followed suit.

But that trend may be short-lived. As the stock market rose strongly Thursday on news that China does not plan to sell off the European debt it holds, for example, mortgage rates edged up by a quarter