  We solve Schr\"odinger equation describing a tunneling macrospin coupled to a
torsional oscillator. Energy spectrum is studied for various quantum regimes.
Magnetic susceptibility and noise spectrum are computed. We show that
entanglement of the spin with mechanical modes of a subnanometer oscillator
results in the decoherence of spin tunneling. For larger oscillators the
presence of a tunneling spin can be detected through splitting of the
mechanical mode at the resonance. Our results apply to experiments with
magnetic molecules coupled to nanoresonators.
