This only started today. I started the car on LPG and heard this loud screaming noise from the boot, which turns out to be the fuel pump. While it makes the same loud noise on LPG and petrol, but shouldn't the pump be off when running on gas?

Depends if a disable relay has been fitted to the system - not all installers fit one.

LOL - got any fuel in the tank?

The Fuel Pump on some conversions runs continuously to circulate fuel to your injectors - saves them drying out and "cooking" crap onto them and making them deteriorate quickly.

Advanced conversions cycle the fuel pump every couple of mins...

.. and yeah some conversions disable the pump. Normally just the injectors are cut though.

ah right. yeah theres fuel in there, not much though. better go fill up.