Engine knock is bad. Most likely a bad bearing (SPUN) on the crankshaft which can happen for several reasons but especially if it ran with no oil or you didn't change the oil over a year and a half (Just kept adding oil as an example) . You get sludge build up, then one of the oil passages gets blocked and the crankshaft or connection rod over heats and spins a bearing.
Now for the news, GM will know when they open up the oil pan if it was due to non oil change or maintenence! Clean inside shows changed oil but if its dark brown or cooked, it shows neglect. You will have to show oil changes (Receipts) by a certified shop, and even then, good luck.
The good news, if the spun bearing is a connecting rod bearing, they can pull the crankshaft, resurface it and put an over sized bearing and replace the connecting rod.
The BAD news, if its a crankshaft journal bearing, well eh, the bearing spinning messes up the engine blocks bearing mating surface on the block and the block must be pulled and probably replaced, new engine.
Happened to one of our work trucks when the guy who had it neglected to change the oil regularly.
The more you drive it the worse it will get and the louder the knock till it gives. I wouldn't start it until you verify if you have oil pressure but by now its probably too late.