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Twice in the last 2 months I have stopped the car after a lengthy drive (100+ kms) and heard a bubbling noise from the engine compartment. On inspection, the water in the coolant tank is bubbling where one of the pipes enters the tank. The noise occurs only intermittently - not after every trip. The first time, the noise abated and stopped after a few minutes; the second time, it stopped when I re-started the engine, and did not recur after the return journey. The coolant is currently on the MIN mark of the reservoir.
Can you advise what the problem might be?
Thanks

It is possible you may have a problem with the engines cooling system. A faulty thermostat, damaged water pump impeller, blocked radiator or even a radiator fan which is inoperative can cause this. Worst case scenario would be a blown head gasket but you haven't mentioned any overheating problem so at this stage it wouldn't seem likely.

Note: There is a slight possibility what you experiencing is normal due to the release of coolant under extreme pressures into the expansion bottle but you need to get the vehicle to a mechanic to inspect the system as there is a chance a problem exists as mentioned above. I suggest you get this checked out by a mechanic as soon as possible.