A person spends a whole afternoon lying beneath a car, and the poem gives us only one reason: to see what sings. That small phrase carries most of the weight. A mechanic would normally listen for a fault, a rattle or a squeal. Here the sound is called singing, as if the machine had a voice worth hearing rather than a problem to be fixed.
The speaker seems patient, absorbed, and quietly content. Nothing is hurried, and "all afternoon" suggests a long stretch of unbroken attention. There's also a nice twist in the verb "see." You'd expect to hear a sound, but the speaker wants to see it, which blends the senses and makes the work feel almost like exploration.
The mood is calm and curious, with a touch of tenderness toward ordinary things. Judging by the title and the wording, the poem treats repair work as a kind of listening, and finds something close to music in a stretch of ordinary labor.