1995 Monte engine knock

I recently bought a 1995 Monte Carlo with 3100 V6 with 120,000 mile. Before buying the car I looked it over and drove it with no problems. Drove the car home with no problems. I did some work to the car and drove it with no problems. Drove it to work the other day and still no porblems. When I went to leave for the day, as soon as I started the car, I had a problem. The car seemed to be running rough, had acceleration problems and a noticeable "hollow popping" noise coming from the engine and the service engine light came and stayed on. The car acted as though it wanted to die at idle, but did not stall. The previous buyer had just purchased the car and didn't know it's history. He seemed like an honest person and told me the few things he had done to the car. I'm thinking I'm the sucker. So far the car has had the following work done. Previous owner had new serp belt, radiator, hoses and battery installed. I changed oil, installed new tensioner pulley, new knock sensor. Fuel pressure is showing 41 lbs. Checked all wires (good), plugs (one was loose, but looked good), tightened coil pack. Haven't driven the car anymore because of the now definite "knock", which seems to be coming from the top of the motor (driver's side maybe, but definitley top). The car starts everytime and seems to idle better now than before, revs ok, but has nasty "knock". I cannot check flash codes because the ALDL plug has only "A" & "M" positions. Please help. I am out of ideas, time and money and think I'm stuck with a piece of crap that I won't even be able to sell. Thanks.