loose rattle noise in engine at start

[nette_282]

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?

[94VL]

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