(hopefully) final update:
I got a call from Hyundai at around lunchtime today and they told me the car would be ready by Friday for collection, and about half an hour later I got another one informing me they will have it ready for me by 3 o' clock!
I got down to Hyundai at around 16:00, and I was out the door with my fixed i30 by 16:30 - rush hour traffic. Scouted out a fuel station as the car was ridiculously empty and drove it to work, after which I went home. So far so good - the service agent did mention I must check for water leaks and the water levels every morning to make sure it is all well and that the gasket has formed a seal. Also, they seemed to have forgotten to bill me for the antifreeze that I was supposed to pay in on - so I informed the service manager who shrugged and said I should consider it a bonus and not worry about it.
So big plus to Hyundai PE for getting my wheels sorted - I would have rated them better if they could have afforded me a courtesy car as I needed one for work, and the turnaround time was a bit drawn out (but understandable due to the third party warranty).
And one last thing to note - no more whirring sound!