[quote name='Madrab' date='08 August 2010 - 12:37 AM' timestamp='1281223623' post='91869']
You may find the pressure cap on the expansion tank is goosed. If it is the cooling system cannot pressurise properly and the coolant will heat up faster, kicking in the fan sooner than it should and more often.
Hopefully you are not getting the coolant/air actually bubbling up through the expansion tank?
after reading the post properly sorry was knackered
you said hissing so i agree with madrab it sounds like you need to get a new pressure cap its either not hiolding pressure ie top rad hose is soft or its overpressurising top hose is hard not holding can be as simple as the cap over pressuring can be down to a faulty cap in the best case or head gasket in the worse