Monitor flickers when crop tool is turned on. Lightroom Classic

Thank you for the tip, fixed the issue.  The GPU was on Auto now it's turned off.

Forgot to add: the monitor is pretty old, got it in 2015. It's a Dell U2713Hb. The new monitor is on backorder ( LG 27MD5KL-B), I hope it will perform well even with the graphics processor turned on. 

First, try disabling GPU in the Preferences (Performance tab). Any better?
If not, recalibrate and build a new ICC display profile, the old one might be corrupted.
If you are using software/hardware for this task, be sure the software is set to build a matrix not LUT profile, Version 2 not Version 4 profile.

If turning OFF GPU works, it's a GPU bug and you need to contact the manufacturer or find out if there's an updated driver for it. On the Mac, that's part of the OS update(s) so if this is the latest OS version, you may need to roll back a release.
Also see: https://helpx.adobe.com/photoshop/kb/acr-gpu-faq.html

It could have something to do with the graphics processor. I just got a new computer, and if I use the super enhance feature the monitor flickers a little unless I disable the graphics processor. But then that affects other things. You might try disabling GPU acceleration and see if that changes things. If it does you'll have to decide whether to try to use it or not.