As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD INFLIGHT T730/28MAY - RETURNED TO ORD DUE TO SMOKE AND FUMES FROM AN UNKNOWN SOURCE. SMELLED OF BURNING RUBBER. LANDED ORD WITHOUT INCIDENT. AIRCRAFT OUT OF SERVICE FOR EVALUATION. ACCOMPLISHED PACK BURN OUT PER WORKCARD 21 92 12. SYSTEM GROUND CHECKED NORMAL OPERATION.