Over time it became a regular, intermittent dripping, or draining sound. As the weather turned colder the peeing became more consistent. We came to refer to it as Bob, peeing, after a long night at The Westminster.
After some investigation we discovered that a bleeder valve, or a modulating valve, "bleeds off" near freezing water from exposed pipes and allows the system to refill with warmer re-supply water. The bleeder valve will close when the temperature of the water is above a set point and the danger of freezing is over. In our case, the bleeder valve started in early October and probably won't shut off until early April.
The bleeder must be in here somewhere. Or Bob.