Why does the spindle sound bad?

If the machine makes any rattling or repetitive (cyclic) noises when it is not cutting, something may be wrong with the bearings, or there may be a misalignment or lack of engagement of gears in the spindle (potentially due to wear). Although these are more long-term issues, it’s good to check what you can control before jumping to conclusions. For example, make sure that the mill is fully in high or low gear rather than somewhere in the middle.

Here’s a video troubleshooting this issue.