Some time ago, I had the dealer replace the oil return line when they did, they spill oil into the #5 spark plug hole, and the car had a slight misfire. I changed the plugs from autolite titaniums to the copper champions (just to see how the copper ones worked)

that was about mid December

about 2 weeks ago I started the car and got a thump (which sounds like a backfire in the exhaust) it happened once and the car set no codes and ran drove fine afterwards, this coincidentally happened about a week after they replaced the lower motor mounts. It's done the same thing a couple times now (always once, and it's a quiet sort of noise)

it has not set any codes

I pulled the #5 and #3 plugs over lunch today to see if I oculd get any insight.

the electrode and insulator all look fine, but there is a ring of carbon around the base of the threads. Is this an indicator of it running rich? an indicator of the copper plugs being junk (I've already ordered OEM volvo ones)

it seems to be burning off the electrode, and only makes the noise on a cold start (is the cold start system creating an over rich condition which causes the mild backfire int he exhaust?)

I don't want to fry the cat to set a code...