What could be causing the humming sound in my hot water radiator?
Sometimes the circulating pump moves too much water and causes noise and vibration. Years ago when I was doing residential service and most pumps were single speed I carried cut down impellers just to fix this problem. Most pumps back then were from Bell and Gossett. Today there are numerous fixes. take the name brand of the pump, the model number and serial number of the pump, (a picture may also be a good idea) to a local hvac shop that services hot water systems. They can look up the flow from a chart and recommend what to do: a cut down impeller, smaller pump,  a multispeed pump etc. This should fix the hum or vibration.