How low can home loans go?  Freddie Mac reported Thursday another historic low in interest rates.  The average rate on  15- year, fixed mortgage fell to 2.875 percent.   Last week it averaged 3.02 percent.  The average 30- year home loan also set a record by dropping to 3.675 percent down form last weeks 3.78 percent.  Frank Nothaft, of Freddie Mac's chief economist, states " it is unreal, isn't it?"  Nothaft states the low rates are caused by the economic woes in Europe, which have pulled down the yield on long - term Treasury bonds.