As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
NOTICED A POPPING SOUND COMING FROM THE BACK OF ACFT. BURNED RUBBER ODOR WAS THEN NOTICED. COCKPIT SUPPLY TEMP WAS THEN OBSERVED RISING IN AUTO MODE. ON TROUBLESHOOTING OBSERVED SMOKE IN THE COCKPIT UPON WHICH THE APU WAS SHUTDOWN. GAIN ACCESS, TROUBLESHOT, FOUND AFT COCKPIT ACM DEFECTIVE, R & R COCKPIT ACM WITH A REPAIRED UNIT. PERFORMED OPS TEST AND CHECKED SATISFACTORY IAW MM 21-51-00.