Inducer fan and blower turn on every few hours, even though heat is off

I have an old goodman gmp075. When the thermostat is set to off, fan on manual, even with thermostat removed from wall-- every few hours, I will hear the inducer fan kick on along with the blower fan. After 20 seconds, the inducer will turn off, and 90 seconds later the blower turns off. Is it the board, or is there something else I should be checking, such as a short in the wires to the thermostat plate?

submitted by /u/thenewyorkgod
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