When remodeling a room on the first floor of my house, I added radiant floor heat to the room. To do this, I added a layer of 3/4" OSB on top of the original subfloor and routed a 3/4" groove to accept 1/2" pex pipe. After installing the pex pipe, I added a layer of 7/16" OSB to the top before installing a laminate floor. This heats well and is quiet. I have now done the same heating arrangement to my second floor. While it heats well, it is noisy. This is a problem because our bedrooms are on the second floor. While the boiler is running, the noise is a nearly constant light popping sound. When the boiler stops and the floor starts to cool down, the noise is less frequent. I attempted to bleed all of the air out of the lines, but do not know if I have been successful. Is air in the lines a possible cause of the noise, or is it more likely that the noise is a result of expansion and contraction of the PEX pipe? Any help you can offer would be greatly appreciated.