Why's my dishwasher making grinding noises during pre-wash?
We have an LG dishwasher as well and it does the same after a few years of operation. Currently its still grinding away but washes well, like you mentioned. I did some research before and it seems that for us its either the:

Circulation Pump
Circulation and Drain Pump Motor

It seems like our source of problem is when water is being circulated and not any drains since towards the end, when the drain is working, little noise is produced.
I have promised myself that I will do some troubleshooting to Identify root cause but haven't done yet.
Source: https://www.repairclinic.com/RepairHelp/How-To-Fix-A-Dishwasher/35-200--/Lg-Dishwasher-Making-Noise