I have a 2002 Lennox Elite series, gas forced air furnace( no A/C ). The first 30 seconds to a minute sounds like a jet engine that will wake the dead. It heats the house fine, the fan is quiet, the filter is not an issue, the fresh air intake has a screen and is not blocked and the sound is not affected by the outside temp. The loud jet turbine is worse when the heat has not been turned on for several hours or over night. The sound has gotten louder as time has gone by. This is my first winter in this house so no past history. Thanks