There is a bypass pipe from the supply to the return on this basement radiator. Apparently the water is whistling by the supply. Are there "Mono-flow" fittings on the supply and return to this register? A monoflow tee fitting has a sorts scoop that catches the water and forces it down to your radiator. You need one on both the supply and return. They have to be oriented correctly on both the supply and return. Water, like electricity, will take the path of least resistance.

Does the radiator heat at all? If theres a lot of air in the pipe you sometimes have to wait a minute. Do you have the makeup water turned on when you're bleeding the air?

I'm probably not going to tell you what you want to hear, but I am going to let you know what you should know: "The technician told me he had checked to see if there was enough water in the boiler, and there was." Now, do you know where the water guage is, the pressure guage and the temperature guage? That is how you tell what is happening in the boiler. You can have burners on and very little water in the system which can lead to problems, serious ones. Every homeowner should know something about their units. To be ignorant is not bliss. Anytime you let air or water or both out of a bleed valve, the pressure in the system drops and you will eventually need to add some water from the inlet valve. This should be marked and off during normal operation. Also, you have an expansion tank? Probably. It needs to be checked also. They get full and then the heated water has no place to expand. You should only need to bleed from a radiator upstairs, not the baseboards although they pronbably have a bleed valve screw also. At the end of the run. Do you know how to know if a radiator top is full of air? By feeling with your hand. Cool top sections may mean air is trapped there and heat doesn't transfer as well thru the air pocket. Do you know if the valves on each radiator are open? Not the bleed valve but the inlet near the bottom of each radiator. They may be 1/4 turn valves, not like on a garden hose valve on the house. You might call the tech back and this time have him show you where things are and what you should be aware of. Did you have a thermocouple changed? (CDM/DMC), is what? What is the psi and temp reading on the guage when the boiler is operating? Should be a guage on the top or side of the boiler. " "Maybe the radiators always did have to fill up each time they kicked in, but I never heard water trickling in before." Think of the radiator system in your car. There is x amount of water and antifreeze in the system. It is always there waiting to extract the heat from the engine block and dissipate it thru the radiator coils. Once the car starts, a pump circulates the water. It is always there, but sitting still when the system is off. A fan is added next to the radiator to speed up the process of dissiapating heat. Also, a heater coil (mini radiator) is installed in the passenger compartment with a fan to use some of that heat in the wintertime to warm the occupants. If too little water is in the system, it overheats, boils, and the excess pressure opens a safety valve on the radiator cap allowing the overheated water/antifreeze to escape. Remember the old pics of the stranded motorists? Your boiler also has a safety valve, and opens at about 30psi. You probably should have about 12-14 psi in the system whether or not the boiler is on or not. You may have a circulating pump on that system also and should know something about it also. As for the trickling noise, I would call the company back and report that there is something different with the system after the tech visited and that he did not fully explain things to you. Try to get a free visit buy you may have to pay something for their time. Remember to ask about the expansion tank. Do you have yearly service?

It's not going to cause damage, the radiator just isn't going to heat as effectively. What's happened is that your system pressure is now to the point where there's not enough water in the system. That's why you're hearing the "trickling" noise. There are a couple of things you need to do. First is to make sure that the expansion tank isn't waterlogged (completely full of water). If it is, it needs to be partially drained, or the air cushion re-established. Next, you need to add water to the system. This may, or may not, be necessary depending on the condition of the expansion tank. The pilot light problem has nothing to do with this issue. Your best bet is to have someone come out, do a full inspection/cleaning on your system, and show you how to check the expansion tank and add water to the system.