What was found
CREW HEARD A LOUD "POP" NOISE FOLLOWED BY AN ACRID ODOR IN THE CABIN . PILOT DID NOT NOTICED ODOR OR HEARD THE POP. PILOT CHECKED THE AIR CONDITIONING, HEATER, CHECKED ALL OF THE INSTRUMENTS. EVERYTHING APPEARED TO BE WORKING NORMALLY. MADE A PRECAUTIONARY LANDING. A COMPLETE VISUAL INSPECTION OF THE ACFT FOLLOWED BY A GROUND RUN DID NOT REVEAL ANY ISSUES. A OPS CHECK FLIGHT WAS COMPLETED WITH NO DEFECTS NOTED. THE ACFT WAS THEN RETURNED TO SERVICE.