so I installed a Cobb downpipe on my '12 wrx a few weeks ago, and after installation, I started it up, and it sounded a bit strange at idle....Sounds like a diesel, fast tapping noise. It only makes this sound at idle though, usually louder on a cold start. I've gotten mixed responses on what the sound is, but I just wanna see if anyone has the same sound. It's a cobb catted downpipe mated to a stock cat back. anyone have any input?