Be careful here: although this instrument is sometimes called a marimba in Spanish-speaking countries (as your text says), it is quite different from the instrument normally known as a marimba, which is like a xylophone, with wooden bars struck with mallets (originally from Guatemala):
http://en.wikipedia.org/wiki/Marimba

In fact I would almost be inclined to put "sometimes called a marimba in Spanish", instead of just "marimba", to avoid confusion.

Be that as it may, a marímbula, a Caribbean instrument with metal strips, is plucked, not struck, and that's what "pulsar" means.
http://en.wikipedia.org/wiki/Marímbula

I don't think there's anything wrong with "at the free end". That's how they express it here:

"The marimbula is an African rooted instrument created by slaves in Cuba. The slaves would create instruments out of any materials that they could get their hands.. The marimbula consists of a small, hollowed box with springy keys or tongues, which are plucked at the free end to produce sound. Most often, the marimbula is used to create the rhythmic bass sound in Cuban music."
http://klovski.blogspot.com.es/2006/11/cuban-music-dance-rhy...