Furnace Blower turns on and off by itself

[Stu79]

I have a gas furnace and blower unit in my attic as well as an AC unit outside. The fan/blower will just turn on and off not activated by the thermostat or the heat or cool settings. Even if I turn the thermostat off from cool or heat the blower keeps running. If I kill the power to the furnace, this will stop the on off cycle. Is there possibly something wired wrong with the thermostat and furnace that could cause this?

Thanks..

[HayZee518]

check the plenum switch. its a box about 3X4 inches with a cover and maybe a manual push-pull manual switch on it. the thermostat actuates 1) the burner relay or 2) the cool relay which turns on the compressor. has nothing to do with the fan unless there is a relay contact on either the cool or heat relay.