Bongo Fury

Over the past few weeks I noticed that my water expansion tank fills up with air and seems to push the waterlevel down. When I open it the waterlevel goes back to the normal as the air can escape.
When I leave the van on idle and check the expension tank I can see little air bubbles rising up.
It looks like the cooling system is sucking in air somewhere without loosing any water.
I can also hear a faint noise from the heater system whilst driving and when I accelerating which sounds like trapped bubbles in the pipe system.
I don't seem to be loosing any water and the temperature is nice and low whilst driving.
I have never changed the coolant and have done around 6000miles with the van.
Never had any overheating issues or waterloss.
So I wonder where the airbubbles come from as I can't see any leaking pipes or radiators etc.
Any suggestions?

I recommend you have the system pressurised with a pump and gauge to check for coolant/pressure loss. This will indentify any pressure loss in the system. then have some pressure in the system by using the handpump and start the engine, letting it warm up. It should not exceed the pressure on the cap (0.9 bar if I remember ) if it does then suspect the gasket.
I hope I'm wrong but it sounds that you may have the early symptoms of a blowing head gasket.................