Navien tankless banging when using washer

[fittan]

Hi, I just moved into house with a Navien tankless NPE-240A. Whenever I start the washing machine, the tankless would make a VERY loud banging noise (see video link below). The tankless works fine otherwise for my showers, etc. It is just this annoying sound when using washer. I have also installed 2 new "water hammer" valve but they didn't help.

Does anyone have any idea what could be the cause? Thanks.

https://www.icloud.com/photos/#005ClSIJJNlhQfm-8YdTeUGRw