I don't know of an established echoic word or phrase for human footsteps. Manga (Japanese comics) translators seem to use "tap-tap-tap" to translate the Japanese words (that language is very rich in onomatopoeia), which is highly unsatisfactory, as all the others that I have seen used. Horses and such have one though, "clippety-clop", which is in dictionaries. I don't think this works for people.
If you really need one, I think you have to be creative.