What was found
(CAN) PILOT SMELLED A STRANGE BURING SMELL IN COCKPIT WHICH LAST APPROX. 30 SEC AND DISAPPEARED. SHORTLY FOLLOWING SHE STARTED FEELING A FLOATING SENSATION /TRANSFERRED CONTROL TO STUDENT/WITHIN 30 SEC THE STUDENT FEELING SAME EFFECTS.THE PILOT THAN DECLARED EMERGENCY AND REGAINED CONTROL FLIGHT WAS IMC. WE OPENED THE STORM WINDOW SHUT OFF HEATER/CHECKED THE CO DETECTIR WHICH INDICATED NORMAL. WITHIN 30SEC WE FELT BETTER WITH THE FRESH AIR AND LANDED WIT HOUT INCIDENT. INSPECTION SHOWED AIRCRAFT HEATING SYSTEM REVEALED A 2.5-3 INCH CRACK IN THE COMBUSTION HEATER CAN, IN THE AFT WELDED LID. HEATER W AS REPLACED. NEXT DUE FOR PRESSURE DECAY TEST AT 500HR. HEATER HOBBS TIME.