Hi everyone!
I experienced AFIB a month ago and was in the hospital undergoing tests for 3 days. When released, one of the meds prescribed was 3.125mg of Coreg. After a week, the dose was increased to 6.25mg. I experienced a single vibration episode once each day the first 3 days, and on the 4th day had 4 episodes, all above the area of the heart. The dosage was reduced back to 3.125mg and the episodes went away.
I don't know if this helps, but it's another data point - if you have had a change in meds recently. I agree - it is certainly unnerving when experiencing it, as I had never heard of these symptoms before.