Recently, a kind of foam metal casing treatment (FMCT) was proposed and tested in a low-speed axial compressor. It was found that the FMCT performed well in both stall margin improvement and noise emission reduction of the compressor. The result is beneficial to the future advanced turbofan design. As a companion research, the goal of this paper is to give insight into the inner working mechanism of the FMCT in terms of unsteady pressure signal analysis across a range of operating points. The investigation is focus on pre-stall behavior of the compressor with/without the FMCT. Pressure measurements using high frequency response sensors have been carried out on the compressor to capture pre-stall process and interpret the flow structure in the blade tip region. A particular emphasis is given on the flow patterns present in the tip region and how they evolve in different casing wall conditions. The results show two different stall inception behavior throughout the throttling process. The compressor with the foam metal placed at the rotor mid-chord experiences a long-length-scale stall precursor. With smooth casing and the foam metal placed near the rotor leading edge, the compressor shows a short-length-scale stall precursor. The relevance of results obtained and possible mechanism involved are also analyzed and discussed at last.