Thank you for your question, my name is ***** ***** I am a fully qualified technician with over thirty years in the trade, hopefully I can help.
If you simply topped the system up it may be that you have a leak somewhere in the cooling system and the noise you are hearing is air which is running through the system. In normal use the cooling system shouldn't drop its level much, and topping up needed would be minimal. If this is the case the source of the leak should be looked for.
If you have drained the system down and then topped it up, then the noise would be the air being dispersed after the drain down and the coolant should settle, but may need topping up again. The cooling system contains many air pockets which can hold small areas of air, this will then as the engine warms up and he coolant starts to disperse the air, create a gurgling noise, this is acceptable.
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