First of all, I'm not a doctor or specialist, so don't regard my post as absolute gospel, always get your symptoms checked out by your GP or Healthcare Advisor. What I say below is only from my personal experience of these palpitations over the last few months and nothing more. It sounds to me like PVC's or PAC's (Premature Atrial or Ventricle Contractions) that I experience on a daily basis. Been happening since December 2012 and have been to A&E too many times to mention. These A&E trips were mostly at the initial stage of the onset of the palpitations due to lack of knowing what they were. I kept thinking it was something really really serious, which to an extent it is, but not life threatening. After many GP visits and a visit to a cardiologist, I have been excluded from any heart disease and assured that the palpitations are benign. Have had bloods done, treadmill stress test, tilt table test and echocardiogram. All come back clear or fine except the tilt table test. They used the tilt table to induce a "panic attack" which they concluded was the reason I was having my palpitations. They said it was stress/anxiety related and I needed to address issues before it would get better. So three to four months later I am still having them daily. Sometimes when I walk, sometimes when I get out a shower/bath, most times through the day for no reason and each night when I'm laying in bed trying to get some sleep. I haven't been able to pinpoint anything that makes them better or worse at this point, so can't say I'm a great help in that sense. What I will say though is that I have tried to understand the anxiety that is supposedly responsible for the symptoms and it baffles me somewhat. I can't pinpoint what my stressors or aggressors are to bring on the anxiety either but I know for sure that the palpitations cause a great deal of the anxiousness I experience. Makes me think of the "chicken and egg" scenario, not sure which came first when I think about it, the palpitations or the anxiety? I feel the thud you mention but also feel what I guess is like a tickle. Both these make me feel a bit dizzy, but it's like instantly there and then gone again, the dizziness doesn't linger. Apparently the thud or tickle depends on where the additional beat happens to come in during the "normal beats" and the thud is due to the heart having a lot more blood in it to pump out while the tickle is when it has very little from what I understand. Apparently this is "all normal" in a structurally normal heart, but each and every time it happens, it still scares the life out of me.
I would suggest you visit your GP for a referral to a cardiologist so that any structural abnormalities can be ruled out. In majority of cases from what I've read or been told, once that has been ruled out, your palpitations will most likely be "benign" and "nothing to worry about" which is seriously easier said than believed. The reality is anything related to the heart scares us and that's why most of us are here, for reassurance and help in dealing with them.
I wish you well in finding an answer and remember to post anything you find that helps or aggravates them, it could be useful to some people. I read on one post that if you are exercising to the extent you are, you could possibly have a low iron level which could cause them, but seeing your GP and having bloods done might tell you if this is the case. Always best to get advice from the GP or from a specialist.
Take care and hope to hear something positive soon