What is the cracking sound when I accelerate? on 2002 Lexus RX300

I drive a 2002 Lexus RX300 and I hear a "cracking" sound when I accelerate. What could be causing this?

Asked by for the 2002 Lexus RX300
exhaust leak. I would guess exhaust manifold has a crack
Thank you! Could a bad O2 sensor cause that as well?