Strange short periodic vibrations on Apple Watch 7 watchOS 9.3

For about half a year I am experiencing strange vibrations on my Apple Watch:

• vibrations are always the same: one short vibration, pause, 2 short vibrations;
• their intensity is somewhere between wheel scroll haptics and "reminder to get up and walk" notification - small enough to not notice it when walking, and a single vibration duration is very short - say 100ms-ish;
• the pause between vibrations is between 2-8 seconds, rarely more;
• they also occur in Sleep mode;
• as for their frequency, they seem to occur at least every hour (mostly at 45-00 minutes), although without a fixed schedule. The shortest period I noticed was ~17 minutes. They also don't happen on 00 seconds.

After searching for similar posts online, I've tested following ideas that didn't work:

• cover watch face to silence notification - this feature is disabled, however vibrations occur both when watch is not covered or was covered for some time;
• chimes - this feature is disabled;
• hidden notifications - I've checked all installed apps and all of them have either fully disabled or fully enabled notifications. These vibrations also occurred when I was actively using phone and no notification popped up;
• notification on watch - nothing appears on screen during vibration;
• two fingers screen touch - doesn't even work for me;
• they are not phantom - multiple times I was able to touch watch with another hand to feel two vibrations after pause.

They are most noticeable at night when trying to sleep, that's how I noticed them in the first place. What could they be?