I posted a thread about this before but now I narrowed down the location of this noisy rattle when I start my car. I removed my rear seats and had a friend start the car with my ear to the back. The tapping / rattling noise is coming from directly under the middle of the rear seats, closer to the trunk. I think it might be the stock UCA/ UCM? Or could it be something in my differential? I think it is just the low idle on a cold start that is causing this to vibrate and make noise because after it warms up the noise is gone. I am changing out my UCA/UCM next week so hopefully that fixes it. Any ideas in case this is something different? Thanks!