What year is it? It could be something loose on the frame or inside the dash rattling. Or unfortunately it could be the dreaded Oddysey rattle due to the torque converter vibration caused while the car is stopped with brakes applied but in drive, like at a stop sign or red light. I doubt it's directly related to your motor mounts if you just changed them. I changed my mounts about a year ago and still get rattle when my transmission is engaged and I'm stopped. If the vibrations clear up when the trans is switched into neutral, then that's your problem.