Combo Well Pump/Pressure Tank used for lawn only. When I shut the power off and the remaining water drains out, the tank make this loud groan type noise which last for up to 10 seconds. When I power the well pump, it starts filling the tank without any noise.
I've tried different settings of bladder air pressure and tank cut in/ cut off pressure settings (which is working fine).
Any idea what's causing this noise - it sounds like the air bladder is vibrating in the tank as the remaining water drains out.