So, at the gas station a couple nights ago, and the car was running at an idle. It was pretty dark out, and I turned and noticed a few sparks coming from the tail pipe. I only did it for a couple seconds, and haven't noticed it since. I'm guessing it's carbon? However, is this normal? What would cause it? The car didn't sound any different than any other time.

 

1973, 240D. Approx. 170k miles