SOME unusual noises characterized as grinding and resonance were reported in the cockpit of a twin-engine jet aircraft. This paper presents a physical model and the related studies on grinding by postulating the noise as due to the vortices shed from the compressor blade tips with markedly excessive clearance between the rotor blade tip and the compressor casing. Using this model an equation has been obtained for the grinding as well as the general compressor noise; the equation accounts also for the resonance and highlights the importance of uniform compressor blade tip clearances within narrow limits to eliminate noise accompanying the blade vibrations. Contents During production flight tests on several twin-engine jet fighter aircraft, test pilots reported occurrence of unusual cockpit noises emanating possibly from the engine and characterized them as grinding and resonance. Discussions with the pilots indicated that the noise was of low frequency; the intensity was marked at higher altitudes and feeble at low altitudes. This noise was imperceptible at low altitudes and engine speeds. Furthermore, vibration of stick, pedals and seats was reported on another aircraft at all altitudes and throttle settings in addition to grinding and resonance at higher altitudes indicating unbalance as causing the grinding, resonance and vibration. Besides alarming and distracting the pilot from his main mission, these unusual noises aggravated pilot fatigue and hence required remedial measures. Ad hoc rectification procedures followed earlier were unsuccessful in the present case. A study of the problem conducted in conjunction with the pilots' comments indicated that the vibration and noise must originate from the compressor dynamic unbalancing factors. Unbalance must be produced by causes having acoustic side effects since dynamic unbalance cannot vary with altitude; although the noise is imperceptible at lower altitudes, it must, nevertheless, be present at all engine speeds, aircraft speeds and flight altitudes including stationary ground running conditions. For this purpose systematic ground and flight investigations were performed on a total of seven engines and two aircraft to understand and identify the causes for grinding, resonance, and vibration. To investigate this problem in depth, an aircraft with the reported defect was equipped with a 0-20 kHz frequencyresponse, sensitive microphone and a tape recorder to record both cockpit noise and pilot comments. In addition vibration transducers were fixed to measure vibration levels. The cockpit noise recording was printed on a trace recorder and