What was found
AT CRUISE FLIGHT, THE PILOT TURNED ON THE CABIN HEAT & APPROX 3 MINUTES LATER, THE MEDICAL CREW NOTICED SMOKE FILLING UP THE CABIN AS WELL AS NOXIOUS FUMES. THE CABIN HEAT WAS TURNED OFF & THE WINDOWS WERE OPENED TO VENT THE CABIN. THE FLIGHT WAS ABORTED & THE ACFT RETURNED TO BASE WITHOUT FURTHER INCIDENT. ONCE ON THE GND, THE MECHANIC ARRIVED TO INSPECT THE ACFT HEATING SYS. AFTER NO OBVIOUS DEFECTS WERE FOUND, A 20 MINUTE GND RUN WAS COMPLETED. THE SMOKE & FUMES COULD NOT BE DUPLICATED & THE ACFT WAS RETURNED TO SERVICE AT THAT TIME.