It's an oil burning furnace that heats water that is pumped to several different zones of the house. As of this morning, it has been making a steady low noise, as well as the regular (normal) much louder noise, when the burner starts and heats water from time to time. Since there are often periods when there is no noise, I'm sure that the steady background noise is unusual. I'm thinking this steady noise might be something to do with the pumping? If it wasn't Saturday, I'd turn off the electrical breaker to the furnace, then restart it, to see what happens, but I don't want to lose the furnace during cold weather this weekend. Any ideas?