Navien Combi boiler NCB-240 intermittent sound

[Greg158]

I occasionally hear this sound from my Navien NCB-240 unit. It makes the sound when it calls for heat or it calls for DHW. But it only happens occasionally. I have been cycling the power to the the unit to get it to stop. It does not throw an error code. (See YouTube link below)

Some specs .. Dual zone baseboard hydronic heat and Domestic hot water unit. Installed in March of 2014. Unit has worked great up until now.

Can someone give me their best guess what this sound indicates?

 

[breplum]

actually a customer of mine had similar but I was never there with the cover off to id the issue. It was age of yours, so we replaced the unit.
remove the cover and see if it is the draft inducer fan. That is the most likely source, but also try to bleed off a little water from the pump...I've seen people report that removing the little black drain port on the underside but can't imagine air being trapped because the air vents should handle that.