  An electro-mechanical setup for the measurement of AC-forces in a
low-temperature tunnelling microscope has been developed, which enables
extremely high force resolution. The crosstalk of vibrations onto the
tunnelling current is used to measure the deflection of a force-sensing
cantilever beam. We demonstrate its capability to measure the noise of the
force at a tunnelling contact using polycrystalline Iridium. Depending on
temperature, spring constant and current, a resolution in the range of $\rm
{fN}/\sqrt{\rm Hz}$ is possible. We observe peak levels of the force-noise at
the energy of the expected phonon maximal density of states, which suggests
that inelastic transport processes contribute to force fluctuations.
