My house is 4 years old. I hear a gurgling sound when the boiler is on and water is running through the pipes. How do I bleed the air from the system?

Since this is pretty new I am assuming you have baseboard radiators. Remove the end cover and inspect the pipe as it comes up through the floor. Where the pipe does 90 degree turn you should see a bleed nipple with a noticeable screw. Keep a rag ready and remember this is very hot water. Counterclockwise turning the screw will allow the air to escape. Close the screw when water starts to come out. How many radiators do you have and do you know which is closest to the boiler, the first one to be fed? Back to you.