I have a 1998 cavalier automatic. Shortly after starting the engine, the car always made 3 knocking noises. I had questioned my mechanic a few years ago as to what that was. He said it was a self test for something (don't remember-abs,tc). In the last couple weeks, when I have started the engine there is a rattling noise like something is loose. It's gone after about 10 minutes, but the 3 knocks are no longer happening. I think it's related. Still trying to pinpoint the exact spot the noise is coming from before taking it to my mechanic.
Does anyone have any ideas or suggestions?

When you start the engine,the vacuum operated heater controls can "activate"...that creates a sound.Also,relays can click too.