I got this when changing the coolant on mine - it was just an air lock. One thing to remember - to get the hot water running thru all your system (inc the heater matrix) be sure to have your heater temperature dial set to HOT. I got the air out by leaving the cap off the exp tank and running till fans came on, whilst 'burping' the pipes by squeezing them, including the 2 that go into the heater matrix.