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Noise from hydronic baseboard heating, when boiler is firing
We've lived in this house for about 5 years, and the hydronic baseboard heating has worked well, and it's been quiet. (There is a little bit of thermal expansion "tinking", but it's completely fine, almost reassuring.) However, this year the system is making a deep rumble-rattle noise. After some observation, it seems to only be happening then the boiler is firing/heating-up.
The full sequence is this...
You're in the living room, at the far end of the house from the boiler. You start to hear a low rumble that slowly gets louder and louder. Some of the baseboards will start buzzing a bit, starting with those furthest away (closest to the boiler), then it works it way to the ones closest (furthest from the boiler). This takes maybe 2-3 minutes. Then suddenly, it stops.
If you stand in the boiler area while this happens, it exactly coincides with the boiler hitting it's low temperature point and turning on, then ending when the boiler hits it upper temp and turns off.
We have 3 heat control loops in the house, and this only happens on one of them. The other 2 circuits remain quiet. The noisy loop is the furthest from the boiler and has the highest elevation.
I've read through some of this forum. I'm thinking the cause may be the expansion tank having lost pressure. I don't see any leaking but at least one of the relief valves dumps directly into a drain so I can't see if it's wet. I'm fine with calling in someone to do service, but I like to have an idea what's happening before I do.
Can anyone help diagnose this? First things you'd check? Thank you!