Repair my car as per my question

my kia carnival 2002 keeps less heating when the car is in slow speed & when it is fast at 100 KM/H the RPM comes down, there is gear oil at Max & engine oil at max and water is getting low. with this i replaced thermostat, have had radiator pressure tested and was fine, radiator water is full but coolant box is getting empty whilst driving the distance of 100 kms. also when it begins to little overheat or just taking off the speed there is a loud ascending rattling noise which seems to come from the under right side of engine But when it catches the speed everything is smooth. & then you park for an hour the coolant tank is empty.or at low. please tell me what to do.

There is a minor leak in the system and this is depleting the coolant from the reservoir.

You need to get the complete system pressure tested. Look out for signs of leakages, especially at hose joints.