As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PUSH BACK, HEARD GRINDING NOISE ASSOCIATED WITH ODOR. SELECTED APU BLEED OFF, WHICH ELIMINATED NOISE AND ODOR. INSPECTED AND FOUND ACM SEIZED. R & R ACM IAW WC 21-50-03, STEPS 13 AND 25. OPS CHECKED GOOD. NO NOISE OR ODOR NOTICED.