There is a muffled rattling sound from behind the dash. I'll post a sound clip when I can. RPM and throttle does not affect it as far as I can notice. It's constant and not erratic. As far as I know it's coming from right in front of me. Sometimes it isn't there when just starting out, but it always comes back after driving a bit. Could it be the exhaust piping right before the cat rattling after heating and expanding?