Good question. Either can be rhythmical. The source of either can be internal (like the heart or music) or external (like a beating). However, "pounding" is used more for external and "throbbing" more for internal sources. "Throb" has the potential to be gentle and loving, whereas "pound" does not. So, for example, you can speak of someone upon whom you have a crush as a "heartthrob", but there is no such word as "heartpound".

These verbs are similar because they both relate to a repeating sound. The main difference is that to throb usually occurs with a regular rhythm, whereas to pound does not need to be rhythmic. For example: "After the race finally ended, he could feel his heart throbbing like never before." "She pounded on his door for twenty minutes, but he wouldn't answer." I hope this helps. Keep up the great work.