Sound of constant running water

I recently moved into an apartment which has a fan coil unit with a pipe for cold and hot water

- Since yesterday, I can hear the sound of running water constantly in the wall unit. The sound continues even when the heat it is off and I turn the thermostat to off

- The unit works fine when I turn it on - the fan comes on and heats the apartment. Once the heating set point is reached, the fan turns off but the sound of running water continues

- I didn't hear this sound for the first 2 weeks in the apartment. This only started yesterday

- Is the hot water only supposed to run when the unit is on heat or is it supposed to flow constantly?

Thanks

source https://hvac-talk.com/vbb/threads/2244094-Sound-of-constant-running-water?goto=newpost