Harmonic imbalance. Get some double sided tape and stick a small weight, (say a very small nut, or even better, the sticky weights used to balance a car wheel) on one of the blades, and see what happens. If the noise ceases, job done. If it gets worse, try sticking it to different blades at different distances from the centre.
Many ceiling fans come with stick-on weights to balance it after installation. Ceiling fans don't resonate, but they will swing wildly if out of balance. Your A/C has a fixed pivot and its inability to counteract the imbalance results in resonance.