problems with coolant system

ok, i seem to have solved my last problem with the loss of power but i now have a new problem that surfaced this week. the cars coolant is disappearing without any visible signs of a leak! i drive it for bout half an hour and the coolant resevoir is virtually empty and was full when i left at the start of the journey. the coolant mixture has turned a rusty brown colour also and i can hear what sounds like what flowing/bubbling from within the cockpit of the car, down towards the passenger footwell. any ideas of what the problem might be and any solutions, cheers adam

ok, i seem to have solved my last problem with the loss of power but i now have a new problem that surfaced this week. the cars coolant is disappearing without any visible signs of a leak! i drive it for bout half an hour and the coolant resevoir is virtually empty and was full when i left at the start of the journey. the coolant mixture has turned a rusty brown colour also and i can hear what sounds like what flowing/bubbling from within the cockpit of the car, down towards the passenger footwell. any ideas of what the problem might be and any solutions, cheers adam

I had same problem, and it turned out to be headgasket gone.
check for sighnes of yellow gunk under oil cap or may be an air lock
try venting radiator when engine is hot, or could be a cracked block.
also check under water pump for leaks.