What was found
BURNING SMELL AFTER APU START, ENGINES SHUT DOWN ON APU BLEED SUPPLY / STARTED APU IAW CRJ900 AMM 49-10-00-866-801-A01 RAN PACKS AT FULL BLAST STILL NO SMELL. RAN BOTH ENGINES WITH PACKS AND RECIRCULATION FAN IAW CRJ900 AMM 71-00-00-866-806-A02. NO SMELL. TAXI AIRCRAFT TO MX HANGAR. WHILE TAXING NO SMELLS. SHUTOWN APU AS REQUIRD NO SMELLS. COULD NOT DUPLICATE FAULT. NO DEFECTS NOTED.