Renault Megane making loud clonking noise when driving?

I noticed a while back that there was a gushing noise coming from the engine (abit like water sloshing) and i ignored it :( Now its starting to make a loud (And i mean loud)clonking/knocking /grinding noise when driving. It isnt loosing any water but it is eating petrol (60 miles on half a tank)

Can anybody suggest what this is so I can get her to the garage??

Thanks

Claire
March 2011