You've come to the right place. From what I understand, you've had an ECG and a 24 hour Holter, in addition to an X-Ray, and your physician told you what you are experiencing are benign. That should be reassuring. Has he/she done blood tests as well, certain electrolyte imbalances can contribute to palpitations, as well as several other things that can be determined by blood tests.
If you continue to have concerns, or a change in symptoms, like additional pain, shortness of breath, etc., talk with your doctor, he/she can suggest additional tests if warranted.
Palpitations are very common, many, if not most, people that have them cannot feel them. One way people find out is when they have a physical screening for a job or pre-surgery, and it is discovered during the examination or ECG.
This is a page by an Electrophysiologist, a doctor that specializes in heart rhythm disturbances. Specifically, he's addressing benign PVCs (Premature Ventricular Contractions), a common form of heart beat irregularity that is, despite how they feel, not a concern, seemingly more so to the doctors then patients like us that can feel them. www.drjohnm.org/2013/06/benign-pvcs-a-heart-rhythm-doctors-approach/
This doctor, speaking of benign PVCs, says more than 20,000/day would be required before he would consider an ablation procedure to correct them. That's 14 per minute 24 hours a day. I recently had an outbreak of PVCs (which I'm still suffering from), this has been happening on and off for 40 years, and although this outbreak is a little worse than I remember, the doctors still feel that they are benign in nature. My Holter, within the last few weeks, showed 6000 PVCs. That would be 4 per minute 24 hours a day. Since I really don't believe I have them while sleeping, that increases the number I have per minute during the day (when averaging).
Beta Blockers slow the heart rate, this helps to reduce PVCs. In addition, Beta Blockers reduce the forcefulness of the contraction of the heart, this helps reduce that annoying thud that we can feel with PVCs. Some forms of PVCs are followed by a compensatory pause, waiting for the next normal beat to occur, the heart filling with more blood during the pause, and when the next normal beat occurs, it's that extra volume of blood being ejected that we feel. Other forms of PVCs have no pause. In addition to PVCs, there are PACs and other forms of premature beats.
Get at least 8 hours of sleep a night, avoid caffeine and chocolate, MSG,and avoid alcohol as much as possible.
Hope this is of some help.