Bilge pump switch/alarm beeps when water pressure pump runs

[LifesBetterWhenYoureBeating]

Hi folks,

My new (to me) boat came with a bilge pump with a switch that has a built-in alarm, but the pump's negative lead was disconnected so it wasn't working. I've connected it to the nearest negative bus (where my mast lights, water pressure pump and others are connected), and it works fine (both the pump and the alarm) but every time the water pressure pump kicks in, the switch emits a short beep. Any ideas what could be the cause of that?

Thanks in advance!