I live in a 100 year old house, and recently noticed a thumping in the radiators. We have radiator heat and the sound usually occurs when the blower comes on. I've tried bleeding the radiators, but to no avail. When I do bleed them, sometimes water and air comes out; sometimes not. The system we have (Weil-McLain, installed 1995) works fine and is serviced twice a year. It works great otherwise, but the thumping is driving me crazy. Does anybody have any advice?