Navien 240 Combi Whistling Noise

Hi all, just installed a Navian 240 Combination boiler/H2O tank on propane. Works great. BUT.. every time it fires up for water or heat, there is a loud, high-pitched resonation. Techs redid the gas piping to increase from 3/4 inch to 1 1/2 inch and (supposedly) checked and set the pressures. I've seen this issue posted a lot (YouTube has a couple of great videos that play the sound exactly) but haven't even seen a solution, rather kind of a lot of 'try this'. Does anyone have direct experience with what worked? It's tough living in the middle of a tea-kettle whistle. :o Many thanks... and all good things to everyone in the New Year.