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About 3 years ago we had the Vitodens 222-F installed to supply DHW and the main floor radiant floor (wood) and lower level Runtal radiators. Everything was fine until I had the installing contractor come out for annual maintenance. It became a circus of leaks, errors and broken parts on what had been a perfectly functioning system. Not long after that I got a recall notice on the heat exchanger so I had them come out in the summer to replace that when it wouldn't be trouble to be without the boiler for a short period. Right after they exchanged the exchanger I noticed a loud humming noise coming from the boiler. I called them back out to look at it and they said it was just because the outdoor reset and indoor temp was such that it was making a pump run because the technician had taken it off of DHW only mode which is where I had it for the summer. Now that we are into the heating season I hear the humming all the time throughout the house. Before the exchange I don't recall ever hearing the system at all unless I was standing right next to it. The pump that seems to be making the noise is located inside the housing, not the circ pumps on the wall. It is the one that draws off the low loss header.
Any ideas what may be causing this noise and how to rectify it?