1990 Buick Electra loud knocking sound

1990 Buick Electra mileage: 100,200. I have loud knocking sound when the throttle is pressed down halfway or more, and the knocking gets louder the further I press the pedal. However, the engine idles fine and accelerates smoothly at low throttle. The car also uses more gas than usual. Using high-octane gas or additives doesn't make the problem go away. I hooked up a code scanner, and the EEC-IV computer shows the following trouble codes: CODE 31: "EGR sensor voltage too low". I changed the sensor and cleared the codes, but it's still showing this code. CODE 41: "Oxygen sensor readings constantly show a lean condition". I also used a sensor tester, and it also shows the oxygen sensors are reading a constant lean condition. I replaced both oxygen sensors, but the engine is still running lean and giving this trouble code. In the past 5000 miles I have: 1) Changed the distributor cap, rotor, spark plugs, spark plug wires, air filter, fuel filter, and battery. 2) Checked the ignition timing and timing advance. 3) Checked for any vacuum leaks. I don't think there are any, because a vacuum leak would make the engine idle too fast, but the engine idles fine. 4) Checked and cleaned the fuel injectors. I used a fuel injector and harness tester, and they open and close properly with no obstructions to the fuel flow. 5) Checked the fuel pressure. It reads 35 PSI, which is within the normal range. 6) ** Hooked up a vacuum gauge. It shows a steady 15 when idling in drive, which seems too low. 7) Checked the Rpms with a tachometer, and it idles correctly at 550 Rpms. 8) Checked the charging system, which reads a constant 13.8 volts. In the past 20,000 miles, I have: Changed the EGR valve, timing chain, gears, camshaft, hydraulic lifters, and alternator. Other observations: I believe the main crankshaft bearings are slightly worn, as indicated by low oil pressure and a knocking sound when engine is first started. Also a slight ticking sound coming from a hydraulic lifter on cylinder #3. I would very much appreciate any opinions, guesses, or even blind hunches from you gentlemen before I take this car to a repair shop.