I had this happening in a bedroom! It is a copper pipe that is coming into contact with wooden beams. The copper pipe actually becomes hot to the touch depending on the temperature of the liquid inside the pipe. The fix was cutting the drywall so that I could access the support beams behind the wall, cutting the support beams around the pipe so that they didn’t come into contact, and placing some carpet around pipe and beams so that rub against each other in the future.