What was found
AT FL400 , LOUDRUSH OF AIR NOISE SIMILAR TO THE NOISE MADE WHEN DOING PRESSURIZATION SYSTEM CHECKS CAME ON CONTINUOUSLY. NOISE CONTINUES NON STOP WHILE ON GROUND, AND A BURNING ODOR NEAR LAVATORY & REAR CABIN. RAN APU & ENGINES WITH BLEED ON, NO DEFECTS OR BURNING ODOR NOTED. WITH ENGINES RUNNING ACCOMPLISHED EMER SYS TEST IAW AMM 21-12-00. TESTED OK, NO BURNING ODOR. ADVISED MCC ON FINDINGS. MCC ADDED DUCT BURN PROCEDURE IAW TC-68A-21-101006M. HEPA FILTER. COMPLIED WITH ENVIRON CONTROL SYS BLEED & TRIM AIR DUCT BURN PROCEDURE.TC-68A-21-181006MR3. R & R HEPA FILTER IAW AMM 21-70-10. OPS CHKD GOOD, AFTER ENGINE RUNS NO BURNING ODOR IN CABIN, UNABLE TO DUPLICATE DISCREPANCY.