Many of the people flocking to adjustable rate mortgages these days may be in for tougher payment burdens than they bargained for, according to a Fannie Mae analyst, as monthly payments could double on some interest-only hybrids once the initial payment structure ends. Noel Fahey, a market researcher and director of regulatory policy for Fannie Mae, said in a new study that borrowers have historically sought adjustable-rate products during periods in which interest rates were…