What was found
AFTER STARTING APU WE SMELL BURNED RUBBER SMEL IT GOES AWAY AND CYCLES BACK ABOUT EACH MINUTE. PERFORMED OPS TEST OF THE LH & RH AIR-CONDITIONING PACK. CYCLED BETWEEN FULL HOT AND FULL COLD TEMP SETTINGS. WENT THROUGH THE CABIN AND FLIGHT DECK TO SEE IF SMELL COULD BE DUPLICATED. WAS NOT ABLE TO DUPLICATE WRITE UP. OPS CHECKED GOOD REF AMM TASK 21-51-03-404-018. OPENED LH & RH AC BAYS AND PERFORMED A VISUAL INSPECTION, NO DEFECTS NOTED.