What could this noise have been?

Last night at around 3:57am, i heard a weird sound in the distance (in the sky). As the object approached my home, it made a sound that resembled plasma at a high frequency. It seemed to slow down and it suddenly started making a weird humming noise, where eventually it flew away from my home. out in the distance, i could hear the humming lower a pitch, (like an engine slowing down). Then i heard it drop about 5 pitches, and it was a low hum, when it shut off. a few seconds later, it started the low humming again, and i eventually couldn't hear it anymore. what was causing this sound?!