In this letter a thermally driven frequency modulated atomic force microscopy (FM-AFM) technique is developed. Thermal fluctuations of the cantilever are employed to estimate the cantilever's equivalent resonant frequency. The corresponding cantilever oscillations are the smallest possible at a given temperature. Related experiments that establish the feasibility of thermally driven FM-AFM in ambient room conditions have achieved tip-sample separations less than 2 nm with long term separation stability (>30 min). Employing this method a narrowband 250 Hz modulation of the tip-sample separation was detected with a vertical resolution of 0.25 Å in a 0.4 Hz bandwidth. The corresponding estimated force sensitivity is 7 fN. In all experiments the cantilever tip was maintained in the attractive regime of the tip-sample interactions. This demonstrates a thermally driven non-contact mode operation of AFM. It also provides a limits of performance study of small amplitude FM-AFM methods.