。
She felt her heart pounding violently.
Literal
She [topic-は] heart [subject-が] violently is-beating [nominalizer-の] [object-を] felt.
The perception construction ~のを感じた ('felt X happening') captures direct bodily experience. 鼓動する (こどうする, 'to beat, to pulse') is the formal/literary verb for heartbeat, and 激しく ('violently, intensely') conveys the force of the pounding. The sentence uses the same perception + の pattern applicable to other senses.