What was found
AIRCRAFT WAS NOT GNDED. ON GND, AT GATE WITH APU ON, NOTICED BURNING RUBBER TYPE SMELL WHEN TEMP TURNED UP WARMER THAN FULL COLD. FOUND DUCT, BETWEEN THE H DUCT MIXING CHAMBER & THE GASPER DUCT COMPLETELY DISCONNECTED, DUCT WAS RECONNECTED. ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK WITH NO MORE ODORS BEING NOTED. ALSO FOUND BOTH RECIRCULATION FILTERS DIRTY & REPLACED THEM IAW AMM 21-24-02. SEE AALA201603099004 FOR ADDITIONAL PART.