Auto home issue on ender 3
It sounds like your z-stop isn't engaging when it's supposed to. The z-axis end stop should be located at the left of the build plate on the z-axis column. When the gantry (with the printhead) comes down, it should contact that. If it doesn't contact it, or if the end stop isn't functioning (isn't sending the signal to stop), the z-axis will just continue to try and go down. Check to ensure it touches where  you want it to touch, and if it is, then check the operation of the end stop. You can do that with a multimeter set to ohms. Just detach the wiring from the main board, place your two multimeter leads on either wire end, then click the end stop. it should go from no continuity to full continuity when you hear the end stop click.

EDIT: The endstop should look something like this: