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I Am having the same issue with my galaxy s20 ultra. I figured out that it is the compass thats having a bug. It reacts randomly to the position in the background even with the sidebar closed, so when you turn north/south/east/west in a certain moment, it vibrates for hitting the direction, like it does when it's open. That's why it vibrates randomly because of movement. When you open the compass in that case, then swipe it away, the vibration stops. Maybe someone else can try this and confirm, so maybe some support staff can pass it to the developers.
Sir, you are a gentleman and a scholar! That is exactly what my s20 ultra was going. I was going absolutely insane with those stupid little vibrations. Before I saw your suggestion, I was able to replicate it multiple times, but I was thinking it had to do with the camera running in the background, trying to focus on whatever I was aiming at. The compass didn't even cross my mind but that is definitely the source of the problem. Mine would go on for hours just vibrating, I was getting to the point where I was about to throw my phone off a bridge and move off the grid, but thanks to you I now know how to end the problem as soon as it starts.
I don't think this is it for my random vibrating. I never opened up a compass on my s20+ and it just happened for the first time TODAY 12/8/2020 (I've had the phone for 6 months). I picked it up, no application, no notifications, still vibrated up until the point I press the lock/unlock button