I think the belt pulley is misaligned be cause it seems to be rubbing the side of the belt.
When I run a wax candle on the side of the belt it temporarily makes the noise stop.
Here’s a video of the problem:
Hello Dodos,
welcome to the forum.
Watching your video many times, it looks like the rear wheel is not inline to the frame.
Red line: Frame axxis (approx.)
Blue line: Wheel diameter
I don’t know whether the angle of sight is the reason, but it would explain that you have to steer slightly to the left to achieve a straight forward moving of your Strida, as it seems below.
Might it be that the rear axle is bent or worn out?