1998 Chevy Lumina Chevy Lumina 1998 3.1L Knocking when pres

Hi all, I hope you can point me in the right direction.

I have a 1998 Chevy Lumina 3.1L, with 195,000 miles (a real mule). I had some pretty significant engine work done about 10 months ago involving the coolant system and valve cover gasket, as well as a replacement of two wires causing engine misfire. Drove fine for awhile, in fact quite well through this past autumn. About two months ago, the service engine soon light came on, but no noticable problem. A month later, I noticed a low banging sound, like a clanging noise that was more pronounced when pressing the gas. No pattern to it, just like something's banging around the inside, I can feel a bit of vibration under my left foot from it. There's no discernable suffering of performance, the gears change smoothly, gas mileage is about the same as it always was, but this damn noise and it's been getting worse (and louder) each week. I'm trying to avoid a code check before I have a broad idea of what's wrong (I'll just junk the car if necessary). Any idea what this might be?