There's a baseboard heating zone in my 2-year old house that makes a knocking noise when the thermostat initially calls for heat; as it eventually stops, I assume that it's the result of the pipes expanding as the water temperature increases. In addition, the knocking lasts longer if the heat has been off for a longer period of time (i.e, first thing in the morning)
I've seen the TOH segment on noise that baseboard systems make but I'm not sure how to address mine since the sound seems to be coming from inside the floor and differs from the normal creaking that aluminum fins make (it's quite pronounced as I can hear it on the other side of the house downstairs)
Also, not sure if this is relevant but I don't think that knocking has always been present; I think that it started last winter and wasn't present the first year that we were in the house
Any thoughts or suggestions?