。
Her rickety bike squeaked and squeaked as she rode down the hill.
Literal
Hill [object-を] go-down [time-とき-に] her [attributive-の] rickety bicycle [topic-は] [mimetic-キーキー] sound [object-を] made.
おんぼろ is a colloquial adjective meaning 'rickety, beat-up, worn-out' — often used affectionately for old possessions. キーキー is the reduplicated mimetic for repeated squeaking/screeching, more intense than single キー. 下りる ('to go/come down') takes を for the path being descended. 音を立てる ('make a sound') is the fixed expression.