1994 Grand Prix over heating

I have a 94 Grand prix V6. I noticed a few weeks ago that the temp would start to climb then drop back to the normal range. I was thinking the thermostat, but the weather got too bad to take in to get fixed. Last week I noticed the heater would not blow out warm air and the temp would stay close to the red zone if I was stopped. This was on a Wednesday. I had an appointment to take it in on Friday. Thursday, it began getting hot again and acting like it was cutting out. I stopped to check the antifreeze. It was empty, but could not find any leaks. I drove a short distance home. Next morning, I started the car and got half a block from the house. The car started the same thing. It made a small knocking sound and white smoked rolled out of the exhaust. I have been given a couple of answers. 1. Threw a rod. 2. The car does not knock until it temp rises. They told me not a rod but too much oil. Oil looks clumpy. And where is my antifreeze going? Other than shooting it. Can you help me?