What was found
NOTICED A FOUL CHEMICAL LIKE SMELL DURING PRE FLIGHT. THE SMELL SEEMED TO BE COMING FROM WEMACS AND GET WORSE THE LONGER THE APU WAS RUNNING. AFTER POWERING DOWN THE AIRPLANE, LEFT BAGGAGE AND DOOR OPEN TO AIR OUT FOR 5 MIN AND THE SMELL WAS NOT AS NOTICEABLE "CORRECTIVE ACTION" REMOVED AND REPLACED CABIN HEPA FILTER WITH A CUSTOMER SUPPLIED NEW FILTER. CLOSED ACCESS. OPS CHECK GOOD WITH NO DEFECTS NOTED. POWERED AIRCRAFT AND RAN APU FOR EXTENDED PERIOD OF TIME WITH NO ABNORMAL SMELLS NOTED. WORK COMPLETED WITH REFERENCE TO AMM 21-70-10.