Abstract. In many X-ray pulsars, a broadening is seen in the wings of the pulse frequency peak which is understood to be a coupling between the periodic variability and the low frequency aperiodic variability. In a transient X-ray pulsar 4U 1901+03, we have found intensity dependent changes in the pulse profile at very short timescales. We attribute the broadening of the pulse frequency peak in this source to these systematic changes in the pulse shape. Similar analysis has been done for several other pulsars to investigate if in all sources the pulse peak broadening takes place due to pulse shape changes. We will present here a summary of our investigation of this phenomenon of pulse frequency peak broadening.