LED flicker when 27k tankless water heater is on

I have a 27k tankless water heater. When it is running, the LED lights in two bedrooms will flicker slightly. And sometimes a bathroom vent light. They do not flicker when my 40 amp EV charger is running, or our mini split.

I'm going to have to hire an electrician to troubleshoot, but I was wondering if there's any other basic things I could check before paying someone to do it.

Some of the things I've already done:

*Ensured all of the nuts on the breakers are tight *Ensure that there is no voltage drop on the lights when the heater turns on *Changed out the light fixture