You can use an electric shaver to make a metallic buzz. I'd experiment with various different shavers as well to get different tones if you can.
When recording them, wave the mic in front of them to get doppler sweeps, layer them randomly to give it movement and while editing them try using various layers of pitch shifting, granulation, comb filtering and maybe a touch of reverb to make some heavy metallic droning sounds.
You should also find several pairs of heavy and light gauge scissors and record the sound of you making a lot of very fast snipping noises so you can also layer those, maybe pitch them up and down and add delays to to get a more precise and detailed metallic wing flapping sound. You'll likely have to roll off the low end of these sounds with an EQ though as you'll already have heavy buzzes and drones from the electric shavers being pitched and layered.
When you're done you can automate volume, EQ, reverb and panning to simulate doppler, proximity and intensity.
Good luck and post the results and how you got there if you can. I'd like to hear what you do with it and what ended up working.