hi ive not long bought a discovery 300tdi,i changed the antifreeze a couple of weeks ago.i flushed it out loads of times due to the water being brown,the missis has been using it with no probs except a gurgleing noise behind dash and the heater being a bit crap.anyhow she gets home from work today and it chucks all the coolant out of the expansion tank.temp was normal tho she said.any ideas please