Bubbling Coolant Tank


I was just checking the levels in the engine today, when I noticed the coolant in the resevior tank had some bubbles coming from the tube from the radiator. I also noticed when I started the engine I could hear water trickling behind the dash.

Its not the head gasket, no smell of exhaust fumes, not overheating, no oil in water, no smoke in exhaust, oil OK, spark plugs OK, rag over exhaust, and I even tried an exhaust analyser at work, which was fine.

It's losing a very slight amount of water, ie. 1/4 bottle has gone down in last 2000 miles or so.

I did have a leak on the top hose a couple of months ago, and replaced clip. I can't see any water leaking from anywhere else.

I checked the carpet where the heater matrix is, and its all dry.

It had a new water pump 3 months ago, along with a new radiator cap.

Could there be a fault with the cap, letting steam escape and air in?

Any help would be much appreciated.

1991 Fuel Injected Sportrak