What is the sound of water running under the dashboard when Mercedes e260l turns on the heater?

Hello, this sound is made when water passes through a warm air tank. Under normal circumstances, there is no sound when water circulates in the engine system. But the sound of running water shows that there is less circulating water, and some air is mixed in it, which is equivalent to the loss of antifreeze. In this case, the owner can add antifreeze first, and then step on the two-foot accelerator to let the engine rotate at high speed and exhaust the air.