Sound of water rushing through baseboard heaters?

Is it really cold outside? Perhaps the system was off long enough that the fluid has some ice chunks in it.

Have you checked the fluid level? There should be a small reservoir near the furnace. In systems I am familiar with, it has a float (not unlike a toilet fill valve) which controls adding more water when it is low. Maybe the water supply is shut off, clogged, or the float is stuck.

Do not—under any circumstances—drain and refill the fluid. It is yucky looking because it is saturated with metals from the pipes and heat exchangers. If you replace the fluid, there will be years of making funny noises and gurgling until it regains equilibrium with the plumbing.

Hot Water Baseboard Heaters

You have air in your pipes, that is what is making that noise. It is not a serious problem and is a fairly simple fix that will require about 10-20 minutes of your time. You need to bleed the air out of the valves. You should have a bleed valve on your radiator that you can open with a screwdriver. Open it slowly until you hear air come out (a small amount of water may come out too so you may want to have a towel handy)- once you see water close the valve. Repeat for any of the radiators that are impacted. Some baseboards don't have bleed valves so this requires you to bleed the air out from water boiler. You should have water valves around the boiler- find the lowest one, slightly open the valve until you can hear some air come out. When bleeding air from the boiler, a small amount of water may come out- that's ok- leave the valve open until the light hissing sound stops. You may need to let some water back in the system after you've bled the air- there is a separate valve that allows water in. Each system is different so consult your manual.

Running Water Sound

That is normal since your heating was base on a hot water running in the pipe through the base radiator. There's no way you can stop that if you need the heat in the house not unless you shut off your boiler and use the electric heater.

from : http://jra813handyman.webs.com/

If you can hear your forced water heater pipes then you have air in them. Find the bleed valve and bleed out the air. (turn off the system and pumps first)

I suspect there is some air in the pipes. I had a house that did that, but I never figured out how to fix it.

I'm having this problem too. What you're saying sounds right, so I'll give it a try!