。
She can't stand that sound.
Literal
She [topic-は] that sound [at-に] cannot-endure.
我慢できない is the negative-potential form of 我慢する ('endure, put up with'); 我慢 itself is one of the cultural keystones of Japanese self-discipline — the virtue of bearing up under hardship without complaint. The に here marks the trigger or target of the endurance: ~に我慢する/できる takes に for what one is putting up with (or failing to). The phrase 我慢できない is the everyday way to say 'I can't stand it' — applicable to noises, smells, behaviors, anything intolerable. Cultural context matters: 我慢 carries positive moral weight, so claiming you can't 我慢 something signals genuine limit, not casual annoyance.