Heart vibrations at night

I feel it vibrate mostly at night, sometimes during the day.  The vibration is on the left side, under the heart and sometimes in the left underarm area.
I take my pulse, it feels regular, and the heart rate monitor says 74 beats per minute.
Had a regular stress test in Dec, and at the end, the doctor said it was slightly different.

What could this be?  I dont want to do a nuclear stress, cause I know the dye is radioactive.

Thanks

[sukumar794]

Prefer a Holter monitor test to validate any rhythmic disturbances. Do consult your cardiologist too.