LPG liquid injection system

I spent nearly $5000 to have a JTG LPG liquid injection system fitted to my 2007 Prado V6, and from the first day it has had a loud noise, like pressure being released, after the engine is turned off. It starts about a minute after turning the ignition off and lasts for about 30 to 40 seconds, and it's so loud my wife can hear it inside our house. The installer said that it was normal and every car he's converted with the system has the same noise. I was surprised that after spending nearly $5000 I have to put up with this. Is this normal? Is there any way I can fix it? Why this wasn't it mentioned to me before I bought it, I would have never converted my car if I knew?