engine knocking

Hello anybody? I am completely stumped. My girlfriend has a chevy venture 99 and when she was driving it today it stalled out and now when it starts it has a huge knocking coming from the engine compartment. It wants to stall out but stays running. It almost sounds like timing or maybe misfiring. It is undriveable right now. The engine light came on a couple weeks ago and read that it was the o2 sensor at the time so she brought it in and fixed whatever they thought it was. O really do not think they fixed anything important. I could really use some help if anyone knows what it might be.