What was found
THE PILOT SMELT WHAT APPEARED TO BE BURNING RUBBER IN THE ACFT RETURNING TO BASE FOLLOWING A PATIENT TRANSFER. THE PILOT TURNED OFF THE AIR CONDITIONING & OPENED UP THE FRESH AIR VENTS. HE THEN DIVERTED TO & LANDED. AT NO TIME WAS THERE EVER SMOKE IN THE COCKPIT. THE MECHANIC WAS CALLED TO LOOK AT THE HELICOPTER. ALL COWLINGS WERE OPENED & THE BELLY PAN WAS DROPPED FOR INSPECTION. A 15 MINUTE GND RUN & 13 MINUTE TEST FLIGHT WAS COMPLETED. WE WERE UNABLE TO DUPLICATE THE PROBLEM & RETURNED THE HELICOPTER TO SERVICE.