As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI CREW REPORTED VERY STRONG BURNING SMELL FOLLOWED BY GRINDING NOISE AND VIBRATION. SHUTDOWN APU AND INDICATIONS WENT AWAY. INSPECTED APU, ENGINES AND AIR CONDITIONING PACKS IN VARIOUS CONDITIONS OF POWER AND BLEED SETTINGS. NO DEFECTS NOTED AND AIRCRAFT RETURNED TO SERVICE.