。
I felt my heart pounding hard.
Literal
I [topic-は] heart [subject-が] intensely beating [nominalizer-の] [object-を] felt.
A perception sentence: [event]-のを感じる ('feel [event] happening'). 心臓が打つ ('the heart beats') with 激しく (adverbial form of 激しい, 'intense'). ~ている marks the ongoing beating. Double-subject construction: 僕は marks the experiencer, 心臓が marks the heart as subject of the inner clause.