Words whose sounds refer to, suggest, or otherwise are associated with a particular meaning are cases of sound symbolism. Although onomatopoeia - direct imitation of a real-world sound - is one type of sound symbolism, it is not the only one.
A common sound symbolism is sound iconism. With the related clustering, this is the re-use of sounds across a set of words with related meanings. Note that this is the re-use of sounds, not morphemes. One example is found in the set of words stamp, stomp, tamp, tromp, and tramp which have the common /-mp/. The final /-mp/ sound is strongly suggestive of stomping or stamping, though it's actually not imitative of it.
Another set of words is glisten, gleam, glint, glare, glam, glimmer, glaze, glass, glitz, gloss, glory, glow, and glitter. The /gl-/ is associated with shining, though it's not imitative in any way since shining is visual, not auditory. Nevertheless English speakers hear these words as related.
This is not unique to English of course. For example, in Japanese linguistics one finds the terms phenomime and psychomime for similar phenomena.