Can it be air in water tank?

Check the oil, if it's whitish or creamy colour. Stop driving the car, you've probably got an head gasket blown, and the oil and water are mixing. The engine would run very hot! the more you drive it the more gunge you're sending around the engine. An expensive job to clean up.
Check the water expansion tank, if there's globules of oil in it, you've got the same problem as above. A garage can test this using a probe, you can't always see this problem.
If you've recently messed with the water in the rad, there might be an air bubble in the system, you'll have to bleed the system to get rid of it.
Driving the car in or out of gear has no effect on the water problem. However good luck, come back to this site to ask for more info if you need it.

Ray Carcel
February 2008