While driving a week before Christmas, I heard a ping and a sound like some sand or gravel fell and bounced against the floor and a fun exhaust leak was heard. I originally thought I lost a bolt on the Y-Pipe I put in. Once I got home, all was well. Other than a big leak between the gasket on the passenger side. Now that I've changed the gasket and still had the leak (I suspect my poor attempts at grinding the surface flat after booger welds screwed up the surface is the issue) and I gooped exhaust sealer around it, the noise was still there. Thinking back, earlier in the spring, I gooped exhaust sealer over the air injecting port that was since been cut and removed is the culprit, think roughly a touch smaller than a dime. It's clearly coming from the cat areas. Manifolds appear tight and sealed, no carbon around the heads or spark plugs/wire boots. Once it's warm enough again I'll dig into it.