My car has a rattling sound when it's cold and during acceleration, but the sound seems to go away once the car is warm. I've had the muffler and catalytic converter inspected, and I was told that both appear to be fine. What might be the problem?

 

welcome to the forum. It may the internal parts of the catalytic or muffler. Suggest you jack the car up and when all the parts are cold, bump or shake the the units. Or get ourself a mechanic's stethoscope (sp)looks like what the doctor uses to listen to your heart. but has a rod at the end. stick it to the cat and you can hear rattling real good,When this parts warm up the expansion quiets down the rattling.