I could hear a still-beating pulse of his heart.
Could it mean two things?
- It is still beating. (still now)
- It is beating quietly.
In this expression, still is the adverb with the temporal meaning: the heart has not stopped beating.
The position of still as a complement of a verb strongly pushes it to be interpreted as an adverb rather than an adjective. Although some informal dialects of English soften the boundary between adverbs and adjectives, the adjective still, meaning “not moving”, directly contradicts beating, so there is no risk of confusion.
There are other expressions about a heart where still can have the “not moving” meaning, for example “be still, my heart”, which is a self-exhortation to be calm. In this expression, still is an adjective (like quiet), not an adverb (like quietly).