Weird rattling, squeaking noise
 

When I start my car everything is fine, no squeaking or rattling or anything. When I drive my car there is kind of a squeaking, almost like a belt intermittently, but not a long drawn out screech or anything. I hear it when i go over bumps, and under acceleration sometimes. After I pull in I can hear the squeaking for about 30 seconds, then it goes away. Noise is towards the rear of the car. Any ideas?

2004 RX8 GT manual transmission

P.S. My check engine light is on from a failed catalytic converter. Bought an agency power mid pipe instead of replacing the cat. Could this be related? it doesn't sound like marbles in a can or anything like that...