As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAKEOFF, CREW NOTED A RUBBER TYPE BURN SMELL COMING THROUGH DUCT SYSTEM. NO VISIBLE SMOKE. ODOR DISSIPATED AT LOWER THRUST SETTING. AIRCRAFT RETURNED TO MDW AND MADE AN UNEVENTFUL LANDING. INSPECTED AIRCRAFT, FOUND A SYSTEM HYD RESERVOIR OVERSERVICED. DESERVICED TO PROPER LEVEL AND PERFORMED ENGINE POWER ASSURANCE RUN PER BOEING M/M.