Highly stressed silicon nitride mechanical resonators, due to nanogram effective mass and high resonance frequency in megahertz, are attractive for various sensing applications [1]. Unlike optical readout schemes, electrical detections of tiny displacements from silicon nitride resonators are more complicated since they require a thin conductive layer covering on the suspended structure to form a capacitive coupling with external circuits [2]. When mechanical resonators are embedded in complex microwave circuits and isolated from passing DC/RF signals, examinations of their mechanical functions become more difficult [2]. It thus becomes essential to develop a detection method that allows in-situ investigations of spatial mechanical vibrations. Here, we demonstrate our recent progress in using a metallic AFM tip in vacuum (710-4 mbar) as a suspended gate to measure a silicon nitride drum membrane at room temperature. The membrane is covered with a thin aluminum layer to form a capacitive coupling scheme. In this detection scheme, the tip is fixed but the sample holder can be moved in X-Y-Z directions. In order to drive the membrane by electrostatic forces, both DC and AC signals are added through the tip. For the detection, we implement microwave interferometry on the tip and readout mechanical displacements through frequency down conversion [2]. Figure 1 shows a 3D spatial map of mechanical responses of the fundamental mode (~8.76 MHz). Based on this platform, we also demonstrate the frequency tunability by DC voltages and mechanical nonlinear dynamics. This platform exhibits potential in characterizing the spatial dependence of mechanical damping effects and can be extended for investigating other mechanical systems driven by electrostatic forces. [1] Yuksel, M. et al. Nonlinear Nanomechanical Mass Spectrometry at the Single-Nanoparticle Level. Nano Lett. 19, 3583–3589 (2019). [2] Zhou, X. et al. High- Q Silicon Nitride Drum Resonators Strongly Coupled to Gates. Nano Lett. 21, 5738–5744 (2021).