I have a Grundfos UP15-10SU7P/TLC Comfort System. I have a few Comfort Valves under sinks throughout my house. I notice a knocking sound at my Comfort valves when timer has the pump “off” and I open hot or close cold. The knocking sound doesn't happen when the timer turns the pump “on” and I open or close hot or cold. Have these Comfort valves gone bad?