I have had my Prusa i3 clone for about two years and it has always made a screeching sound when the z-axis is moving down. I have tried many solutions for this problem like greasing the z-rods, printing a new top part (the holder at the top z-rods end) with a bearing to prevent the threaded rod from swaying, and even hot glueing bearing to the stock top piece. However, none of this has worked. I am not sure if it is affecting the print quality but the sound is very annoying when it is doing a lot of hops and I would like to fix it. Here is a video (with sound) of the rod moving up and down.