This is pretty much what the wife's 2.0d X3 was doing and pressurising the water system, that was head gasket, and also it's pretty much the same as my wrx is doing now although I have more frequent smaller bubbles and some presssure in the water hoses, and uses a little bit of water, I put some kseal in it and it's not gotten any worse over a couple of months, just gonna nurse it along until I get the other engine finished, but I presume it is head gasket