After washing the bike a couple of days ago, it went from super silent to making this constant weird zing-zing-zing fairly loud noise, mostly under motor power. A similar, but much fainter sound seemed to come from the front wheel as well. Bikes sounds can "telegraph" all over the place, but it was definitely coming from the rear wheel. I knew the rear disk brake was warped a bit, so I thought it was rubbing. I took out the calipers, the brake pads and cleaned everything, including sanding down the pads a bit (they had become glazed). Noticed one piston in the rear was not coming out anymore (managed to fix that as well). All this thanks to youtube! Looking over videos on the processes, I familiarized myself with bike hydraulic brakes as well. It works the same as a car, except for the master cylinder. No reason to bleed the brakes unless you know or suspect that air got in the sealed system. Works best with a bike bleed kit and some specific mineral oil for bike hydraulic brakes. Do not used DOT car liquid as some suggest. Best not to mix different types either. There is a bleed valve on the brake handle (to add oil and prevent air going in with a reservoir that you screw in) and one on the caliper to bleed from there.
I also learned how to fix a warped rotor - I was sure at first only a replacement could fix the issue. Again, super easy with no special tool required.
I conclude anyone can service the brakes on those bikes with youtube help if need be, including bleeding/changing the oil. The calipers and pads are super easy to work with.
Anyway, after doing all this (and checking all the wheels spokes as well which could create such noises), the bike noise had not improved a bit. I thought then that it must be the motor, which was quite disappointing.
But then I remembered something: when I washed the bike at first, I moved the orange wheel reflectors to a different position to try to prevent them flying off like others experienced. Out on a whim, I put them back the way they were and went again for a test drive. Low and behold, the noise was gone... I spent 2.5 hours working on the brakes when it was the reflectors all along, but learned how to service the brakes and rotors from A to Z in the process. Quite satisfying all in all. I still can't figure out how the placement of the deflectors caused such a loud weird noise (they weren't rubbing anywhere), but it also explains why I was hearing some weird sound from the front wheel as well.
Lesson learned gentlemen, lesson learned!