Despite a recent recovery in housing, new trouble in the U.S. mortgage market may be just over the horizon.

Reuters reports on looming problems with bubble-era home equity lines of credit:

The loans are a problem now because an increasing number are hitting their 10-year anniversary, at which point borrowers usually must start paying down the principal on the loans as well as the interest they had been paying all along.

More than $221 billion of these loans at the largest banks will hit this mark over the next four years, about 40 percent of the home equity lines of credit now outstanding.

For a typical consumer, that shift can translate to their monthly payment more than tripling, a particular burden for the subprime borrowers that often took out these loans. And payments will rise further when the Federal Reserve starts to hike rates, because the loans usually carry floating interest rates.