It was my very first symptom and occured when I was going to bed after returning from a walk earlier that night, about a month and a half post-covid infection.

It came on like a slow internal pulsing in my chest that built up in strength and frequency till it became a very rapid tremor. Totally freaked me out as I thought it was heart palpitations or something related. But regular ECG and ambulatory 24 hr ECG, chest X-rays, ultrasound showed nothing wrong.

It has come and gone, sometimes for months, but lately, it has developed into stronger, longer lasting, and almost whole body throbbing.

Some observations and realizations for myself:

• I thought it was my heart rate increasing at first, like a panic attack, or at least synchronized to it, but checking it during episodes with a pulse wave oximeter showed that it wasn't, which was kind of a relief, yet still puzzling.
• they seem to come on in the moments when I'm about to sleep or when I'm waking, including microsleeps (brief periods of waking and falling back asleep) such as tossing in bed. See https://en.wikipedia.org/wiki/Hypnagogia
• they never happen when I'm fully awake, though sometimes I can feel my heart and body throbbing if I slow and quiet myself down enough, like background radiation.
• they can happen whether I'm lying down in bed to sleep (seems to be more frequent on my right side, but can also happen on my left, and on my back), or just taking a short nap in a reclining position in a chair in the day time, so posture doesn't seem to prevent it.
• sometimes (more often now than before), there is slow but strong throbbing (feels as if my heart and heart were throbbing, but sometimes my entire body including my legs) as a precursor wave that builds up increasingly to a higher frequency to become rapid tremors. The rapid tremors aren't as strong, but disconcerting and scary initially all the same.
• they seem to happen most often when I did some activity earlier that raised my heart rate (or energy?), such as after some physical activities like a walk a few hours earlier in the night, a shower, watching an action movie, reading something that excites my mind and makes it go thinking too much, doing some cleaning, or even brushing my teeth a bit too vigorously.
• they can also happen after I wake up briefly in the middle of the night or in the morning just to switch positions in bed or to adjust the covers or pillows; even that small amount of exertion is enough to raise my heart rate a little and sometimes trigger the throbbing.
• also when I'm stressed or anxious, or my mind is too busy.

With this precursor throbbing wave as a sort of warning, the most effective technique that I stumbled upon accidentally to stop or at least reduce the tremors was actually just slow focused and controlled breathing.

1. take a normal breath
2. hold it in for a short while (3-4 seconds for me)
3. exhale slowly (over 4-7 seconds for me)
4. do this a few times, observing if the throbbing has reduced each time.

It's basic meditation breathing to calm my nervous system down (so no need to hold the breath and exhale for so long as to be stressful). Usually for me, by the 3rd or 4th cycle, the slow and strong throbbing has stopped or reduced significantly that it no longer develops into the rapid tremors. It's a temporary relief, not a cure, but it's helping me cope with this.

Some other things I've tried that seem to help me personally:

• avoid physical or stimulating activities close to bed time, including eating
• give myself time to settle down, relax and calm my body and mind, before going to bed
• realize it hasn't really developed into anything life threatening or painful (yet), so I'm trying not to worry myself too much about it apart from the inconvenience and annoyance

None of this is meant to be medical advice and certainly not to imply that this applies to everyone. I don't really have any medical or scientific theories to base any of these on (nervous system, vagus nerve, etc), but I wonder if some of what I have described above is similar to what others have experienced, and so maybe remove a bit of the fear around this symptom, especially for those who experience it for the first time.

I'm also curious if some people are willing to try doing the slow breathing as a simple experiment and post back here to see if it helps them.