High pressure in cooling system?

["Craig D. Niederst" <niederst@telerama.com>]

I've noticed in the past months that my 100 seems to have fairly high
pressure built up in the cooling system, even after the car is cooled down
somewhat. Gurgling sounds can be heard from around the radiator and other
cooling system components, even when the engine is only slightly warm. If I
open the coolant expansion tank, coolant rushes out. The car does not
overheat at all, even in the hotter summer months. Bad thermostat? Bad water
pump? Any ideas? On a related note, its time for a cooling system
flush/fill. Any opinions on the best phosphate-free antifreeze to use as a
replacement? TIA.

Craig
'92 100S (76k)