Generally speaking, yeah, single "thuds" are safe if you've been checked out and there's not underlying medical problem. Most of the population gets them at some time or another, some people notice it more than others.
After I posted this thread I started searching for stuff on the net out of random curiosity. Run a search on google for 'heart sounds' and you'll get a link to a site called assuculation assistant (sp?), it has all the normal and abnormal heart sounds on it. Doesn't help much with describing the feeling, but if you're randomly curious like me you can pick up stethescopes at a lot of pharmacies fairly cheap. It's kinda neat to compare what your heart sounds like when it's making those wierd feelings, to what the site says those sounds indicate.
Of course this is in NO WAY a substitute for medical evaluation!