My house is 5 years old. I have no bleeders on my hot water baseboard heaters. There is a rattle during circulation and it seems to be coming from a Taco 219 swetchek. Is this a bleeder? Is air causing this rattle or could it be something else? This problem does not happen all the time but is often. I am somewhat confused that there are no bleeders anywhere on the heaters. Is this normal?

Is this a zone system? What position is the thumbscrew in?

It is a two zone system. One floor house. Not sure what your asking about the thumbscrew though. A brief description of what you need and I could give you an answer.