The detection of variable emission lines in the 4-9 keV energy band (usually identified with Fe K line complex features) in radio quiet AGNs X-ray spectra is drawing the attention on the possibility of using their properties to map the accretion flow in the strong gravity regime of the black hole. We carried out a model independent spectral variability study of a sample of 11 radio quiet AGNs in the 4-9 keV energy band. Using XMM-Newton data we mapped the emission in excess of the continuum in the time-energy plane. For IC 4329a, the brightest source of the sample, we present a more detailed analysis of the intensity modulation characterizing the detected feature in the E=5.4-6.1 keV range. A total of 5 out of 11 sources show significant variability signatures between 5.4-7.2 keV. In the case of IC 4329a, the detected variable feature is characterized by intensity modulations on a time scale of 32.5 ks, significant at about 96% confidence level, revealing a complex variability pattern. The line modulations apparently lag behind the short-time scale continuum (E=0.3-10 keV) variations by about 15 ks.