I have something similar, but there's a good chance that it might be something called Wolfe-Parkinson-White syndrome, where basically I have a sort of second pulse, that means I feel like I'm having palpitations sometimes. Thing is I don't get it that badly, and all the blood tests and ECGs I've had show up negatively, but that's mainly because I never have an "attack" (for want of a better word) when I'm strapped up the the machine. It sort of feels like my heart is beating more than it should, but not necessarily harder, and sometimes like my chest is a little tight, almost like someone pressing on it gently.
If you go to the doctor and say your heart is playing you up they'll definitely look into it - I only mentioned my palpitations in passing and they got me in for loads of tests the same week.