BMW E46 coolant loss problem

Please help
My e46 coolant warning light comes on,water loss seemed to be coming from the expansion tank which i have now replaced i have also replaced the thermostat, there is no sign of water in the oil so i have ruled out the headgasket
any ideas ?
thanks

try and replaceing the radiator cap on eirly models thay had problems with them not opening and reliseing pressure with would blow lines or usually the exspantions tank itself.