Was the car using much coolant before
you forgot to replace the coolant bottle cap?
The clinking noise you describe - that may be 'pinking'.. a noise caused by an incorrect air/fuel mix .. and also a symptom of a blown head gasket. Another symptom of a blown head gasket is white smoke (steam...) from the exhaust.
First, check your engine oil on the dipstick. If the oil is a creamy grey colour this indicates a blown head gasket. The creamy grey colour is caused by coolant finding its way into the oil system.
Fill up your coolant bottle with coolant. Leave the cap off, and start the engine and let it tick over for 10 minutes. As the engine warms up air will escape from the coolant level - keep topping up as necessary.
Keep your eye open for extreme
bubbling in the coolant bottle - violent
bubbling in the coolant bottle is caused by exhaust gases finding their way into the cooling system via a defective head gasket.
If there's no violent bubbling, replace the coolant bottle cap when the engine has ticked over for a few minutes..
... and then begin to look for leaks as the engine ticks over. Look under the car to see if there's any falling fluid. Check around the radiator and the hoses. Check that the coolant bottle itself isn't leaking.
You say your car isn't overheating - that's a positive. One symptom of a blown head gasket is overheating.
However, your coolant is going somewhere and you should try and find out where ... If it isn't a leak from somewhere, it may well be the head gasket.
Prices? To have a cylinder head gasket fixed costs around £500 - £1000 British pounds. A coolant bottle costs around £30 (look on Ebay)
If the head gasket is leaking/defective you can try a modern sealant/repair, which I am told are now quite good. Have a look on Ebay:
3 229results for cylinder head sealant