This will allow you to test a specific frequency that you hear when you do your resonance testing in Klipper and potentially track down where extra peaks are coming from. If something is rattling at a specific frequency, you can specify that frequency and feel around the printer until you track it down.
Code was adopted by zifnab from somewhere in Klipper.
You have to have [resonance_holder] in your printer.cfg.
The command is HOLD_RESONANCE AXIS=<axis> FREQ=int SECONDS=<seconds>