Oi! Thanks for your replies.
It's a bit weird indeed the bubbling but I thought it was something with Subaru's - a lot of people over here (NZ) are driving Subaru Legacy's and so many of them made that funny bubbling that I actually started thinking that is was 'normal'...
The engine is a small EJ18 one with about 230.000KM on the odometer. I definitely don't want anything to happen with the head gasket. I have no idea how difficult it is to replace it in a Subaru but I replaced one in a VW LT28 in the Netherlands and that was not funny... but well, that's quite a different car
A leaky head gasket leads to quite a notable decrease in engine power doesn't it? I don't think it's the case with my Subaru...
Does the radiator not have a pressure relieve valve what might have caused the steam coming out? On the other hand, if it was such a valve, the steam would've come out in bursts and not a steady slow 'coffee steam rate', wouldn't it?
Thanks again!