Just moved into a home built in 1952 and the cast iron baseboards are great...but they are noisy. I have 3 zones (basement (zone 1), main living area (zone 2), and bedrooms (zone 3)). I have bled each zone, first at the furnace, and then at each baseboard coin bleeder, so I am fairly confident they system is free of air. Each baseboard is getting warm as well. When the heat kicks on, there is 20 minutes of time where there is a lot of popping and snapping in zone 3. The other two zones are noise free. The system is a monoflow hydronic system, so each radiator has a supply and return teed off of a the main supply line in each zone. The master bedroom has a long run (about 37ft) of radiator along 3 walls. I believe the noise is coming from the expansion and contraction of the radiators+copper pipe. I removed the metal aesthetic covers at each corner and on the end of the runs, and noise was reduced significantly. My question is what are my options for "absorbing" the expansion and contraction in the noisy radiator runs? I have seen copper expansion pipes, but have no idea who makes them or where buy them. Any ideas?