On March 13, 1996, at about 1647 eastern daylight time, a Cessna 152, N6174Q, registered to Alternate Flying Club, operated by Fort Myers Airways Inc., as a 14 CFR Part 91 instructional flight, made a precautionary landing in the vicinity of Flamingo, Florida. Visual meteorological conditions prevailed and a VFR flight plan was filed. The airplane nosed over on soft terrain on landing roll out and sustained substantial damage. The student pilot reported no injuries. The flight originated from Marathon, Florida, about 37 minutes before the accident. The student pilot stated, he was in cruise flight at 3,500 feet when the aircraft engine began making a loud continuous noise. He made a precautionary landing, and the airplane nosed over during the landing roll out. Examination of the airplane, revealed the number 3 exhaust tube had separated below the weld joint. There was no evidence of a precrash mechanical failure or malfunction of the engine assembly. When the student pilot was informed of the source of the loud noise, he stated, he used improper flight planning and decision when he misdiagnosed the perceived emergency, which resulted in an unnecessary precautionary landing. An NTSB Pilot/Operator Aircraft Accident Report was mailed to Fort Myers Airways Inc., on March 14, 1996. The certified letter was received on March 22, 1996. No reply was received by the NTSB.