I'm not sure why. We have forced hot water heating, and when the heat turns on, i can hear the water in the pipes all over the house. Is this normal? Shouldn't the air vent on the furnace be letting the air out so it doesn't run through the pipes?
The air vent on the boiler, leaces very little air out.
If you have a steel expansion tank, good chance it is coming close to being water logged, and needs drained.
Then your baseboard should be purged, and your water pressure brought back up to 12 to 15PSIG.