i was wondering if any one knew why my exhaust sounds like its leaking out underneath the car, and why a stock exhaust sounds so deep when its idleing, it also buzzes a little bit past 5000rpm

the muffler was replaced by the people who owned it before me,but it is an OEM muffler so it shouldnt change the sound.also it was in Bartow so the cat may be messed up (i havent done emissions yet)
any help or advice would be great.