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Navien NCB-240E boiler makes vibrating noise on startup and 5-10 minutes after hot water shuts off
We have a new Navien NCB-240 in our new construction home. We use it for hot water and for hydronic heating. It worked perfectly fine for several months, but now started making loud vibrating sounds on start up, and - much more bothering - some 5-10 minutes after I turn off hot water. Basically, I turn on hot water (even form some 30 seconds), then turn it off and the boiler starts making this vibrating noise... It does not happen when the boiler is used for heating. I.e. as soon as the thermostat switches heating to 'system idle' - the boiler gets absolutely quiet almost immediately. But again, after using hot water - it makes this vibrating noise.
I noticed that - when it makes this noise - there is some kind of whistling sound around intake / exhaust pipes. So, I guess it moves air through this pipes, even though it is not actively heating at this moment.
Another symptom that might or might not be related is that hot water temperature setting is 125F, but I noticed that when we run shower, the current temperature indicator consistently shows 131F. Basically, it kind of "ignores" 125F settings and heats water to 131F. But it is not an issue by itself. This strange and new noise - is what really annoying...
Thank you in advance for your help!