Hi. I have a Goodman GMPN Condensing Gas Furnace. The circulating air blower is on constantly, whether the furnace is heating or not. This is a new occurance. Typically, the blower is only running while the unit is heating. Not constantly, as it is now, thus blowing cold air when the heater turns off. Any suggestions on what might be going on, and what I can do to fix it?