Thank you for the replies.

Yesterday before I went out I thought I would check the coolant level as I was told to do this, and I noticed the level had dropped about 1cm below the max (it was always on the max when I have checked it before).

Then as I pulled away I noticed the same water poured/trickling/sloshing into water noise... I then pulled over and checked the coolant level again, and it was now back to the max line...

No idea if this is coincidence or if this helps isolate any potential problem at all?