Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 3000; Flight Crew Type 150
Aircraft Equipment Problem Critical
Flight Crew Executed Go Around / Missed Approach; Flight Crew Returned To Departure Airport; General Maintenance Action
Aircraft
Flight was an instrument training flight conducted in a Piper Arrow II; (PA28R) with one student and instructor.  During IFR training we heard a loud noise sounding like a very rough engine occurred without any additional vibration or visual signs. The instructor took control of the flight and told the student to remove his vision restriction device. After checking with the student to verify the sound was indeed present and appeared to be coming from the engine the instructor informed ATC and; being in VFR conditions; cancelled the IFR flight requesting return to [departure airport]. All engine gauges appeared normal. ATC asked if [it was an emergency] and the instructor stated 'not at this time.' The instructor then request frequency change to [the Tower] and was approved. Contact was established with the Tower. [ATC] requested 'souls on board' and 'fuel on board.' These requests were responded to with 'two' and 'four hours' respectively. [Tower] cleared [us] to land and it was completed without incident.The engine cowling was removed by the instructor and student after parking to determine if there was in fact any damage. Visual inspection of the engine showed that the right front cylinder exhaust stack had broken off of the engine and was at the bottom of the engine compartment. Also; damage to the inside of the cowling was observed; apparently caused by the direct impingement of the exhaust.  No other damage was noticed.
A PA28 pilot and instructor noticed a strange noise but noticed no abnormal engine indications. After a successful return to the departure airport; a broken exhaust stack was noted.