What was found
DURING FLIGHT, AN UNFAMILIAR ODOR WAS PRESENT IN THE COCKPIT. CREW DESCRIBED IT AS A BURNING RUBBER ODOR. ODOR WAS SHORT LIVED AND DISSIPATED SHORTLY AFTER TURNING OFF AC COMPRESSOR SWITCH. ODOR DID NOT RESURFACE AFTER SWITCHING AC SYS BACK ON. DUE TO CLOSE PROXIMITY TO HOME BASE, RETURNED FOR FURTHER INSPECTION. DEFICIENCY WAS ANNOTATED ON 134A, CONDUCTED VISUAL INSPECTION OF BELTS AND ELECTRICAL WIRING WITHOUT RESULTS. GROUND RUN WAS PERFORMED, ALL SYS ACTIVATED WITHOUT RESULTS OR DUPLICATION OF ODOR. ACFT RETURNED TO SERVICE. CHECKED ALIGNMENT OF AIR CON BELT IAW CURRENT AMM 21-51-10,4-1B, PERFORMED GENERAL VISUAL INSPECTION OF WIRING, AND PERFORMED GROUND-RUN TO CHECK AIR CON SYS IAW CURRENT AMM 21-51-10,5-1. NO DEFECTS WERE NOTED AND THE ACFT WAS RETURNED TO SERVICE.