Steam radiator with identity crisis (noisy, spitting water)

Hi all We came back from a couple of weeks away to find out that a couple of our one-pipe steam radiators are having an identity crisis. They are making a loud sound, almost like a washing machine (water churning). Difficult to pinpoint exactly where it’s coming from within the radiator or pipes. Also, they have “spat” some of the lovely, brown radiator water out of the air vent. We recently had the boiler converted from oil to gas, but can’t see how this would be the cause. Any thoughts? Am I totally wrong on that? Could it be an obstruction stopping the condensate draining? (Is it normal for the water to be brown) Or pitch issues? Something else entirely? Thanks AussieBen