The period where borrowers could lock in a 3.5 percent rate on a 30-year fixed-rate mortgage may have ended. Mortgage rates are showing signs of inching up.

Read more: Borrowers Leery of Higher Rates

Rates are "definitely in panic mode," Matt Weaver, senior mortgage loan originator with PMAC Lending Services, told CNBC. "A lot of refinance clients are moving to locks immediately because the Fed talk is starting to be an eye opener for everyone."

Weaver says there has been a rush to lock in rates before they edge higher after mortgage rates, which can fluctuate every day, spiked last Wednesday. Mortgage rates loosely follow the yield on the 10-year Treasury.

"If the Wednesday surge of Treasury yields persists, the impact on mortgage rates is likely to result in a bout of affordability shock to many housing markets across the country," says Len Kiefer, deputy chief economist at Freddie Mac.

The 30-year fixed-rate mortgage is up three-eighths of a percentage point since mid-May, averaging about 4.125 percent.

While three-eighths may not sound like much of an increase, "it's more of what it's going to look like, where we're going. Is it a train that’s not stopping?" Weaver says. "When we see an eighth, a quarter, now it's starting to become typical language. We would normally see a little bit of a pullback, and then it goes up again, and now that's not happening. We're slowly steadily increasing."

Source: CNBC