What was found
SEA - FLIGHT 2399 - FAINT EXHAUST ODOR COMING FROM THE VENTS. SMELLED THE SAME WHEN THE BLEEDS ARE TURNED ON TOO SOON AFTER ENGINE START. FLIGHT CREW SUFFERING FROM HEADACHES AFTER OPERATING AIRCRAFT. NO VISIBLE SMOKE. MAINTENANCE LOOKED AT THE CDS FOR FAULTS AND FOUND NONE. CHECKED THE OIL LEVEL, NO DEFECTS. RAN ENGINES ONE BLEED AT A TIME, THEN BOTH ON AND NOTED NO ODORS. CHECKED WITH ONCOMING FLIGHT CREW THAT NO ABNORMAL ODOR WAS NOTED. AIRCRAFT RETURNED TO SERVICE. NO PARTS REPLACED.