We install the ch240 ng all the time. A few after a few weeks make a loud whistling noise when the burner ramps up. With tech support checked air to gas reading ok. New air sw i had this time. They authorized to change unit. Anyone have an idea. Its loud whistle.
I would say say the tech line at navien would be another place to get answers if we cant help. I had a cus tell me it sounds like a train when they use their navien,and of course it didn't happen when I was there checking their furnace. I called the tech line off their website and now I have a list of a bunch of tech help lines in my work truck. To your whistle I may of checked the flow sensor which can get debris which could be the sound, but I would check other replies to your post.