1990 Daihatsu Charade 'uses' water with no apparent leak

Hi guys,
The car "uses" water somehow, with no apparent leak. The radiator
is a bit rusty but doesn't leak. I aslo had the hoses changed recently.
There is a bubbling noise from under the bonet, near the front of the passenger seat.
I have to keep adding water to the reservoir every 15km otherwise the car overheats. Aslo for the first time yesterday: I heard water gushing out 1' after I switched the engine off, rashed out, saw the water leaking from somewhere near the bottom of the engine, not from the radiator; by the time I got under (2' later), everything was dry and couldn't locate the exact leaking spot.
I noticed some oil-mud like stuff on the radiator cap. No water is escaping into the engine.
Do you think there is another problem apart from a worn out HG?
thanks a lot