As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: DURING DESCENT INTO CLT FLIGHT DECK ODOR DETECTED. MIX OF BURNING RUBBER/ELECTRICAL. ISOLATION ATTEMPTS ALTERNATING PACKS 1 AND 2 OFF THEN ON DID NOT HELP. ODOR IMPROVED AFTER LANDING. MAINTENANCE FOUND THE SOURCE OF THE ODOR WAS THE AVIONICS VENT FILTER AND REPLACED IT PER AMM 21-26-43. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED AND NO ODORS WERE NOTED.