Ok then I'm very curious about this onomatopeia thing. According to Akiji's guide the right sound effect for heart thumping in Chinese is "peng peng" and I guess that's what Chinese mangaka (or manhua authors, whatever the word) normally use in their works to express someone's excited feelings. But the writing of the Chinese language isn't based on phonetic symbols; in fact using Roman letters or Japanese kana for sound effects would totes make sense as those are phonetic symbols. So I wonder, when a Chinese writer decides to go national and stick with "peng peng" or other onomatopeia which are specifically Chinese-sounding, how do they do it? Perhaps use a random ideogram with a random meaning but which sounds the right way?
Every Chinese character has a sense and a sound. By "sense" I mean the idea or the thing symbolized by the character. By "sound" I mean the way it is pronounced.
You can use Chinese characters in a textual way, to form sentences and express your thoughts, or you can use them as onomatopoeia, to phonetically imitate sounds of noises from the world. If it's the latter, then you don't need to pay attention to the sense of the character, as only the sound matters.
I know of two ways to suggest the noise of a heart beating fast: 怦怦 (pēng pēng) and 扑通扑通 (pūtōng pūtōng). The first one (pēng) can be translated as "thumping" or "palpitating" and is used in normal speech to describe a beat-like sound; thus it makes perfect sense as a Chinese equivalent to ba-thump ba-thump or doki doki. The second one (pūtōng) has become very popular lately among comic artists who seem to think it's closer to the sound of actual hearbeat... however in this case you must ignore the meaning of the two characters, as it has little to do with hearts or beating noises